Q.1. According to ancient philosophers, matter consists of: (a) Three constituents (b) Four constituents (c) Five constituents (d) Six constituents
Q.2. Dry ice is: (a) Solid ammonia (b) Solid carbon dioxide (c) Solid sulphur dioxide (d) Normal ice.
Q.3. Which of the following statements is not correct for liquid state? (a) Particles are loosly packed in the liquid state (b) Fluidity is the maximum in the liquid state (c) Liquids can be compressed (d) Liquids take up the shape of any container in which these are placed
Q.4. Which of the following will sublime? (a) Common salt (b) Sugar (c) Camphor (d) Potassium nitrate
Q.5. When the liquid starts boiling, the further heat energy which is supplied: (a) Is lost to the surrounding as such (b) Increases the temperature of the liquid (c) Increases the kinetic energy of the particles in the liquid (d) Is absorbed as latent heat of vaporisation by the liquid
Fill in the blanks:-
1. Matter is made up of small_________. 2. The forces of attraction between the particles are _______ in solids, ______ in liquids and _________ in gases. 3. __________ is the change of gaseous state directly to solid state without going through liquid state, and vice versa. 4. Evaporation causes __________. 5. Latent heat of fusion is the amount of heat energy required to change 1 kg of solid into liquid at its ________. 6. Solid, liquid and gas are called the three _______ of matter. 7. The smell of perfume gradually spreads across a room due to ______. 8. Rapid evaporation depends on the ______ area exposed to the atmosphere. 9. As the temperature of a system increases, the pressure of the gases ______. 10. As the volume of a specific amount of gas decreases, it’s pressure _______. 11. As the temperature of a gas decreases, I’s volume ______. 12. Gas molecules at higher temperatures have more _______ than at cooler temperatures. 13. A sponge has minute ________, in which ________ is trapped. 14. The pressure inside of a sealed tube if you raise the temperature go ______ 15. Forces of attraction in liquids are _______ than in solid. 16. Latent heat of ________ is the amount of heat energy required to change 1 kg of solid into liquid at its melting point.
Very Short Answer Questions
Q.1. Name one property which is shown by naphthalene and not by sodium chloride.
Q.2. A rubber band changes its shape when stretched. Can it be regarded as solid?
Q.3. Gases can be compressed but solids cannot. Explain.
Q.4. Kelvin scale of temperature is regarded better than the Celsius scale. Assign reason.
Q.5. What happens to the heat energy which is supplied to the solid once it has started melting?
Q.6. The freezing point of water is 0°C. What is the corresponding temperature on the Kelvin scale?
Q.7. Are the melting point temperature of the solid state and the freezing point temperature of the liquid state of a substance different?
Q.8. A substance is in liquid state at room temperature and changes into gas upon heating. What will you call its gaseous state?
Q.9. When a crystal of copper sulphate is placed at the bottom of a beaker containing water?
Q.10. The boiling point of ethyl alcohol is 78°C. What is the corresponding temperature on kelvin scale?
Crossword Puzzle
Across 1. BEC stands for Bose-Einstein-______
3. The state consists of super energetic and super excited particles
8. Conversion of solid to vapour is called ______ Down 2. This is the phenomenon of change of a liquid into vapours at any temperature below its boiling point 4. SI unit of Temperature 5. CNG stands ____ natural gas 6. It is the amount of water vapour present in air. 7. LPG stands for ______petroleum gas.
Improvement in Food Resources refers to the methods aimed at increasing the quantity and quality of food produced through agriculture and animal husbandry to meet the growing demand for food sustainably.
As living organisms, food is essential for our growth, health, and overall development. We obtain food primarily from plants and animals through agriculture and animal husbandry. With India’s growing population, the demand for food is rising, and it is crucial to find ways to increase food production efficiently.
Various Food Resources
This chapter focuses on:
The need to improve crop and livestock production to meet food demand.
Successes like the Green Revolution and the White Revolution boosted food and milk production.
The importance of sustainable practices in agriculture and animal husbandry to protect natural resources.
The connection between food security, increased production, and access to food.
Scientific methods like mixed farming, intercropping, and integrated farming improve yields without harming the environment.
Improvement in Crop Yields
Food Crops and Their Nutritional Benefits
Cereals such as wheat, rice, maize, millets, and sorghum provide us with carbohydrates for energy.
Pulses like gram (chana), pea (matar), black gram (urad), green gram (moong), pigeon pea (arhar), and lentil (masoor) supply us with protein.
Oilseeds, including soybean, groundnut, sesame, castor, mustard, linseed, and sunflower, offer essential fats.
Vegetables, spices, and fruits deliver a variety of vitamins and minerals, along with small amounts of proteins, carbohydrates, and fats.
Fodder crops like berseem, oats, or Sudan grass are grown for livestock feed.
Different Types of Crops in India
In India, the production of food grains has increased fourfold from 1952 to 2010, while the area of land available for cultivation has only risen by 25%.
Factors Affecting Crop Growth
Different crops need specific climatic conditions, temperature, and sunlight duration (photoperiods) to grow and complete their life cycles.
Plants gather energy through photosynthesis, which relies on sunlight.
During the Kharif season (June to October), crops like paddy, soybean, pigeon pea, maize, cotton, green gram, and black gram flourish.
In the Rabi season (November to April), crops such as wheat, gram, peas, mustard, and linseed do well.
Major Activities for Improvement:
Crop Variety Improvement: Enhancing crop types to maximise yields, ensuring high production under diverse conditions.
Crop Production Improvement: Implementing methods to increase overall crop output.
Crop Protection Management: Taking measures to protect crops from damage and loss.
Crop Variety Improvement
Methods for Crop Variety Improvement
Breeding for beneficial traits: Selecting varieties with qualities such as disease resistance and high yields.
Hybridisation: Crossing genetically different plants, which can be intervarietal, interspecific, or intergeneric.
Genetic modification: Introducing specific genes to achieve desired traits in crops.
Try yourself:Which type of crops are grown for livestock feed?
A.Cereals
B.Pulses
C.Oil seeds
D.Fodder crops
The factors for which variety improvement is done are:
Higher yield: To boost crop productivity per acre.
Improved quality: Quality requirements vary by crop; for instance, baking quality is crucial in wheat, while protein quality is essential in pulses.
Biotic and abiotic resistance: Creating varieties that can withstand diseases, insects, drought, salinity, and extreme temperatures.
Change in maturity duration: Shorter periods from sowing to harvesting enhance economic viability, enabling multiple crops per year.
Wider adaptability: Developing varieties that can thrive in various environmental conditions stabilises crop production.
Desirable agronomic characteristics: Traits like height and branching are advantageous for fodder crops, while shorter plants are preferred in cereals to minimise nutrient consumption.
Crop Production Management
In India, as in many other agriculture-focused countries, farming varies from small to very large farms.
Consequently, farmers have differing amounts of land, resources, and access to information and technology.
Financial conditions significantly influence the farming practices and technologies that farmers can adopt.
Cultivation methods and crop yield depend on factors like weather, soil quality, and water availability.
Since weather conditions, such as drought or floods, can be unpredictable, varieties that grow well in various climatic situations are beneficial.
Similarly, varieties that tolerate high soil salinity have also been developed.
Nutrient Management
Plant Nutrients
Essential nutrients: Plants need nutrients from air, water, and soil for growth. Air provides carbon and oxygen, water supplies hydrogen, and soil provides the other thirteen nutrients.
Macro-nutrients: These are nutrients required in large amounts, including carbon, oxygen, hydrogen, and others from the soil.
Micro-nutrients: These are needed in smaller quantities and are also supplied by the soil.
A lack of these nutrients can negatively impact plant growth, reproduction, and disease resistance.
Deficiency of these nutrients affects reproduction, growth, and susceptibility to diseases in plants.
Manure
Manure is an organic substance made from the decomposition of animal waste and plant materials. It is rich in organic matter and essential nutrients, which help improve soil fertility and support healthy plant growth.
Benefits of Manure:
Enriches the soil by adding organic matter and nutrients.
Improves soil structure, which: 1. Increases water retention in sandy soils. 2. Enhances drainage and reduces waterlogging in clayey soils.
Environmentally friendly: It recycles biological and farm waste, reducing the dependence on chemical fertilisers and helping lower water pollution.
Manure
Types of Manure
(i) Compost and vermicompost:
Composting: This involves breaking down waste like animal dung and vegetable scraps to create nutrient-rich compost.
Vermicomposting: In this method, earthworms are used to speed up decomposition, resulting in vermicompost.Vermicomposting
(ii) Green manure: Plants such as sun hemp or guar are grown before planting other crops. When these plants are turned into the soil, they enrich it with nitrogen and phosphorus.
Let’s Revise
Q: How does manure improve soil fertility?
Ans: Manure enriches the soil with organic matter and essential nutrients, improving its fertility.
Q:Why is green manure used before sowing crops?
Ans: Green manure adds nitrogen and phosphorus to the soil, enhancing nutrient availability for the next crop.
Fertilisers
Fertilisers are commercially manufactured nutrients that provide nitrogen, phosphorus, and potassium.
They are essential for promoting the healthy growth of leaves, branches, and flowers, leading to robust plants.
Fertilisers play a major role in increasing crop yields, especially in intensive farming.
Careful application of fertilisers is crucial, including the right amounts, timing, and following guidelines before and after use for full effectiveness.
Excessive watering can wash fertilisers away, leading to water pollution.
Repeated use of fertilisers in one area can harm soil fertility since the soil’s organic matter isn’t restored.
Green manure, made by growing specific plants and turning them into the soil, enriches the soil with nitrogen and phosphorus.Fertilisers
Benefits and Considerations of Fertilisers
Boosts plant growth: Fertilisers deliver key nutrients like nitrogen, phosphorus, and potassium, fostering healthy growth.
Increases yields: When used correctly, fertilisers can enhance crop productivity in high-cost farming.
Soil health impact: Regular use of fertilisers can deplete soil fertility as it does not replenish organic matter and can harm beneficial soil microorganisms.
Proper Application and Precautions
While aiming for maximum crop yields, consider the quick benefits of fertilisers alongside the long-term advantages of manure for soil health.
Fertilisers need to be applied carefully, focusing on the right amount and timing to ensure they are fully used by plants.
Organic farming is a method that minimises or eliminates the use of chemical fertilisers, herbicides, and pesticides. It maximises the use of organic manures, recycled farm waste such as straw and livestock dung, and employs natural agents like blue-green algae for creating biofertilisers. Additionally, neem leaves or turmeric can be used as natural pesticides during grain storage, promoting healthier farming systems.
Try yourself:
What are the factors that influence crop growth?
A.Climate, temperature, and photoperiods
B.Soil quality, water availability, and weather conditions
C.Seed selection, crop nurturing, and crop protection
D.Fertilizer response, disease resistance, and product quality
Organic Farming
Organic farming reduces or eliminates the use of chemicals like fertilisers, herbicides, and pesticides.
Instead, it focuses on maximising the use of organic manures, recycled farm waste (like straw and livestock excreta), and bio-agents, such as cultures of blue-green algae for biofertilisers.
Additionally, neem leaves or turmeric can serve as bio-pesticides for grain storage.
Healthy cropping systems, including mixed cropping, inter-cropping, and crop rotation, are used to enhance control of insects, pests, and weeds while providing essential nutrients to crops.
Try yourself:
What is the purpose of using fertilizers in farming?
A.To speed up the decomposition process
B.To produce nutrient-rich compost
C.To promote plant growth and increase crop productivity
D.To enrich the soil with nitrogen and phosphorus
Also read: NCERT Solutions – Improvement in Food Resources
Irrigation
Proper irrigation is very important for the success of crops. Most agriculture in India relies on rain, meaning the success of crops in many areas depends on timely monsoons and adequate rainfall throughout the growing season. Therefore, poor monsoons can lead to crop failure. Ensuring that crops receive water at the right times during their growing season can boost the expected yields of any crop.
Irrigation
Types of Irrigation Systems
Wells: There are two types of wells: dug wells and tube wells. A dug well collects water from shallow water-bearing layers, while a tube well can reach deeper water sources. Pumps are used to lift water from these wells for irrigation.
Canals: This is usually a large and complex irrigation system. Canals receive water from one or more reservoirs or rivers. The main canal splits into branch canals, which have further distributaries to irrigate fields.
River lift system: In areas where canal flow is inconsistent due to insufficient reservoir release, this system is more effective. Water is taken directly from rivers to support irrigation in nearby areas.
Tanks: These are small storage reservoirs that collect and store runoff from smaller catchment areas. New initiatives to increase available water for agriculture include rainwater harvesting and watershed management. This involves creating small check dams, which help raise groundwater levels. The check dams prevent rainwater from flowing away and reduce soil erosion.
Droughts occur due to a lack or irregular distribution of rain. Drought poses a risk to rain-fed farming areas, where farmers rely solely on rain for crop production. Light soils hold less water, and in such areas, crops are negatively affected by drought conditions.
Try yourself:In which irrigation system is water directly drawn from rivers?
A.Wells
B.Canals
C.River lift system
D.Tanks
Cropping Patterns
Cropping patterns refer to various methods of growing crops to achieve the best results.
Types of Cropping Patterns
Mixed cropping: Growing two or more crops at the same time on the same land, such as wheat with gram, wheat with mustard, or groundnut with sunflower. This method reduces the risk of disease and helps protect against crop failure.
Inter-cropping: Planting two or more crops together in a specific pattern, like rows of soybean alternating with rows of maize, or finger millet (bajra) with cowpea (lobia). This ensures that each crop has different nutrient needs, optimising nutrient use and reducing pest and disease spread.
Crop rotation: Growing different crops on the same land in a planned sequence. The choice of crops depends on moisture availability and irrigation. Proper crop rotation can allow for two or three successful harvests in a year.
Crop Protection Management
Field crops face many threats from weeds, insects, and diseases. Managing these threats timely manner is crucial to avoid significant crop damage and losses.
Weeds: Unwanted plants in the fields, such as Xanthium (gokhroo), Parthenium (gazar ghas), and Cyperus rotundus (motha). They compete for resources, reducing crop growth. Therefore, removing weeds early is essential for a good harvest.
Insect pests: They harm plants in three main ways: by cutting roots, stems, and leaves; by sucking sap; and by boring into stems and fruits, all of which can lower crop health and yields.
Diseases: Caused by pathogens like bacteria, fungi, and viruses, which can spread through the soil, water, and air, leading to reduced crop health.
Let’s Revise
Q:What is inter-cropping? Name a crop combination used in intercropping.
Ans: Inter-cropping involves growing two or more crops in a definite pattern, such as alternating rows. Soybean and maize is a common inter-cropping pair.
Q: How do insect pests damage crops?
Ans: By cutting parts, sucking sap, or boring into stems and fruits.
Methods of Control
Pesticides: These include herbicides, insecticides, and fungicides, which are applied to crops or seeds and soil. However, overuse can be harmful to various plants and animals, and it can lead to pollution of the environment.
Mechanical removal: This involves physical methods, such as manual weeding, to manage weed growth.
Preventive measures:
Seed bed preparation: Properly preparing the seed bed and timely sowing of crops can help prevent weed growth.
Intercropping & crop rotation: Planting multiple crops together or rotating them can lessen the impact of pests and diseases.
Resistant varieties: Using crop varieties that resist pests and diseases.
Summer ploughing: Deep ploughing in summer to eliminate weeds and pests, reducing their effect on future crops.
Try yourself:
What is the main objective of organic farming?
A.To maximize the use of chemicals in farming practices.
B.To minimize the use of chemicals and maximize the use of organic manures and bio-agents.
C.To promote the use of chemical fertilizers and pesticides for higher crop yields.
D.To rely solely on chemical fertilizers and pesticides for crop protection.
Storage of Grains
Storage losses in agricultural products can be quite high. The causes of these losses are biotic factors—like insects, rodents, fungi, mites, and bacteria—and abiotic factors, such as improper moisture and temperature in storage. These issues can lead to:
Decline in quality
Weight loss
Reduced germination
Discolouration of produce
Poor marketability
Control Measures
Preventive Measures
Preventive and control measures are implemented prior to storing grains.
Thorough cleaning of the produce is essential.
Proper drying should be done in both sunlight and shade.
Fumigation with pest-control chemicals is necessary.
Effective seedbed preparation, timely sowing of crops, intercropping, and crop rotation assist in controlling weeds.
Utilising resistant varieties and summer ploughing can help eliminate weeds and pests.
Systematic Warehouse Management
This is vital for minimising storage losses.
The process involves organised treatment of grains and effective management of warehouses.
Animal Husbandry
Animal husbandry is the scientific management of livestock, which includes activities such as feeding, breeding, and controlling diseases.
This type of farming involves animals like cattle, goats, sheep, poultry, and fish.
As the population grows and living standards improve, the demand for milk, eggs, and meat also increases.
Moreover, there is a heightened awareness about the humane treatment of livestock, leading to new considerations in farming practices.
Therefore, there is a need to enhance livestock production to meet these rising demands.
Cattle Farming
Cattle farming serves two main purposes: producing milk and providing draught power for agricultural tasks like ploughing, watering, and transporting goods.
In India, cattle are classified into two species: Bos indicus (cows) and Bos bubalis (buffaloes).
Female animals that produce milk are referred to as milch animals, while those used for labour are known as draught animals.
The amount of milk produced is influenced by the lactation period, which is the time after a calf is born during which the mother produces milk.
This production can be enhanced by extending the lactation period.
Exotic breeds (like Jersey and Brown Swiss) are often chosen for their longer lactation periods, whereas local breeds (such as Red Sindhi and Sahiwal) are known for their strong disease resistance.
Cross-breeding these types can result in animals that possess both desirable traits.
Animals like cows and buffalo need clean shelters for their health and to ensure they produce clean milk. They should be kept in well-ventilated sheds that protect them from rain, heat, and cold. The floor of the shed should slope to stay dry and make cleaning easier. Cattle Farming
Feeding Requirements
Dairy animals have two types of nutritional needs: (a) maintenance requirement, which keeps them healthy, and (b) milk-producing requirement, which is essential during lactation.
Animal feed consists of (a) roughage, which is high in fibre, and (b) concentrates, which are lower in fibre but rich in proteins and nutrients.
Cattle require a balanced diet that provides all necessary nutrients in the right proportions. Certain feed additives containing micronutrients can also enhance the health and milk yield of dairy animals.
Diseases and Control
Cattle suffer from various diseases that reduce milk production and may cause death.
Healthy cattle eat regularly and have normal posture.
Parasites can be external (causing skin diseases) or internal, like worms affecting the stomach and flukes damaging the liver.
Bacterial and viral infections also occur, for which vaccinations are given.
Let’s Revise
Q:What are milch animals?
Ans: Milch animals are female animals that produce milk.
Q:Name one exotic and one local breed of cattle.
Ans: Exotic: Jersey; Local: Sahiwal.
Poultry Farming
Poultry farming is the practice of raising birds like chickens for their eggs and meat. Special poultry breeds are created for specific roles: layers produce eggs, while broilers are bred for meat. Cross-breeding between local Indian breeds, such as the Aseel, and foreign breeds like the Leghorn aims to create new varieties with desirable characteristics, which include:
High numbers and quality of chicks;
Dwarf broiler parents for commercial chick production;
Ability to adapt to hot weather;
Low maintenance needs;
Smaller egg-laying birds can thrive on cheaper, more fibrous diets made from agricultural by-products.
Egg and Broiler Production
Broiler chickens are raised specifically for meat and sent to market when they are ready. Their diet is rich in protein and contains sufficient fat. The poultry feed includes high levels of vitamins A and K to promote optimal growth and efficient feed usage. Care is taken to minimise mortality and maintain the quality of feathers and meat.
Importance of Good Management Practices
Good management practices are crucial for successful poultry production. This includes:
Maintaining the housing at the correct temperature;
Ensuring the environment and feed are clean, which involves regular cleaning, sanitation, and disinfection;
Preventing and managing diseases and pests through appropriate vaccination which can stop infectious diseases and reduce poultry losses during outbreaks.
The housing, nutritional, and environmental needs of broilers differ somewhat from those of egg layers.
Broilers have different housing, nutritional, and environmental needs compared to egg layers. Their specific diet is designed to support their growth requirements.
Disease Prevention and Control
Poultry can become ill due to:
Viruses
Bacteria
Fungi
Parasites
Nutritional deficiencies
Regular cleaning, sanitation, and disinfectant spraying are essential.
Vaccinations can help prevent infectious diseases and reduce poultry losses during outbreaks.
Farm animals receive vaccinations against major viral and bacterial diseases.
Poultry in India is the most efficient at converting low-fibre food (not suitable for human consumption) into nutritious animal protein.
Try yourself:
What are the two main purposes for which improved poultry breeds are developed?
A.Egg production and meat production
B.Egg production and dairy production
C.Egg production and wool production
D.Meat production and wool production
Fish Production
The main aim is that fish serves as an affordable source of protein in our meals.
Fish production includes various species, such as true finned fish and shellfish like prawns and molluscs.
Fish can be obtained in two ways: through natural resources, known as capture fishing, or through farming, referred to as culture fishery.
Fish are found in both seawater and freshwater environments. Marine fisheries and inland fisheries are two major sources.
Marine Fisheries
India’s marine fishery resources include 7500 km of coastline and the deep seas beyond it.
Popular marine fish found in these waters include pomfret, mackerel, tuna, sardines, and Bombay duck.
Marine fish are caught using various fishing nets from fishing boats.
Yields are increased by locating large schools of fish in the open sea using satellites and echo-sounders.
Some marine fish of high economic value are also farmed in seawater, including finned fish like mullets, bhetki, and pearl spots, shellfish such as prawns, mussels, and oysters, as well as seaweed.
In such a system, a combination of five or six fish species is used in a single fishpond to avoid competition for food.
Aquaculture in Seawater
Certain valuable marine fish are grown in seawater through a method called mariculture.
Examples of farmed finned fish include mullets, bhetki, and pearl spots. Shellfish like prawns, mussels, and oysters are also cultivated.
Seaweed is another product obtained from mariculture, and oysters are cultivated not only for their meat but also for pearl production.
As stocks of marine fish are depleted, the demand can only be met through cultured fisheries, a practice known as mariculture.
Let’s Revise:Besides meat, why are oysters cultivated?
Ans: For pearl production.
Inland Fisheries
Inland Fisheries
Freshwater resources consist of canals, ponds, reservoirs, and rivers.
Brackish water areas, like estuaries and lagoons, are also important for fish.
While fish are captured in these inland waters, most production comes from aquaculture.
Aquaculture in Freshwater Systems
Freshwater aquaculture involves raising fish in resources like canals, ponds, reservoirs, and rivers. Fish can also be cultivated in paddy fields, where they grow alongside rice crops, making efficient use of water and land.
An intensive method used in freshwater aquaculture is composite fish culture, where both local and imported species are reared together. In this system, fish species are carefully selected to avoid competition for food by occupying different zones of the pond: – Catlas feed at the surface, – Rohus feed in the middle layer, – Mrigals and Common Carps feed at the bottom, – Grass Carps consume aquatic weeds.
This arrangement ensures that all available food in the pond is utilised efficiently, resulting in a higher overall fish yield.
However, a major challenge in this system is that many fish species breed only during the monsoon. Additionally, fish seed collected from natural sources may get mixed with other species, affecting the purity and quality of the stock.
To address this issue, scientists have developed methods to breed fish in ponds using hormonal stimulation, which ensures the year-round availability of pure and healthy fish seed in the desired quantities.
Challenges in Fish Farming
Many fish types in composite fish culture only breed during the monsoon season.
A significant issue in fish farming is the shortage of quality fish seed.
To solve this problem, methods have been developed to breed these fish in ponds using hormonal stimulation, ensuring a reliable supply of pure fish seed in the required amounts.
Bee-Keeping
Bee-keeping for honey production is a farming activity. Besides honey, beehives also provide wax, which is used in many medicinal products.
Local bee varieties, such as Apis cerana indica (the Indian bee), A. dorsata (the rock bee), and A. florae (the little bee), are used alongside the Italian bee (A. mellifera) for commercial honey production.
The Italian bees have high honey collection capacity, sting somewhat less, and stay in a given beehive for long periods.
The quality of honey depends on the pasturage, and bee farms or apiaries are established for commercial honey production.
Honey production also provides wax, which is used in various medicinal preparations.
Factors Affecting Honey Quality
The quality of honey relies on the availability of flowers that bees can gather nectar and pollen from.
Besides having enough flowers, the types of blooms influence the flavour of the honey.
Italian bees are known for their high honey production. They are less aggressive, remain in the same hive for extended periods, and breed effectively
Sound is an essential part of our everyday life, coming to us in many different forms. But what is sound exactly?
Sound is a type of energy that creates a sensation of hearing. It is made by vibrations and travels in waves. It is important to understand that sound waves are a kind of mechanical wave, which means they need a medium (like air, water, or solids) to move through.
Key Characteristics of Sound
Nature of Sound: Sound is produced by vibrations and travels in waves.
Transmission: Sound requires a medium (such as air, water, or solids) to travel.
Loudness and Intensity: Loudness is how our ears respond to the intensity of sound.
Audible Range: The typical range of hearing for most people is between 20 Hz and 20 kHz. Sounds below this range are called ‘infrasonic’, while those above are called ‘ultrasonic’.
When you clap, you create a sound. But can you make sound without using any energy? In this chapter, we will explore how sound is made, how it travels through different mediums, and how it is detected by our ears.
Production of Sound
Sound is created by objects that vibrate. It is a type of energy that we hear with our ears. When objects vibrate, they generate sound waves made of compressions and rarefactions, which are key to understanding how sound moves through the air. A compression is a part of the sound wave where the pressure is higher, while rarefaction is where the pressure is lower.
Activity:
Vibrating tuning fork just touching the suspended Table Tennis ball
Objective: Observe how vibrations create sound and influence nearby objects.
Materials: Tuning fork, rubber pad, small ball (table tennis or plastic), thread, needle.
Procedure:
Strike the tuning fork against the rubber pad to make it vibrate.
Hold the vibrating fork near your ear and listen to the sound.
Touch a vibrating prong with your finger and feel the vibrations.
Hang a small ball using a thread. Lightly touch the ball with the vibrating fork and watch how it moves.
Observations:
The vibrating fork makes sound.
Touching the prong lets you feel the vibrations.
The ball moves when it is touched by the vibrating fork.
Conclusion: Vibrations create sound, and sound can also be produced by plucking, scratching, rubbing, blowing, or shaking different objects.
Sound can be generated through various actions that cause objects to vibrate. Vibration means the quick back-and-forth movement of an object. The sound of a human voice comes from vibrations in the vocal cords. When a stretched rubber band is plucked, it vibrates and makes sound.
Compression is the area of high pressure in a sound wave.
Rarefaction is the area of low pressure in a sound wave.
In brief, sound is made by vibrating objects, and grasping the ideas of compression and rarefaction is important for understanding how sound travels through different materials.
Try yourself:What is vibration?
A.The production of sound through striking objects
B.The motion of an object from side to side
C.The rapid to and fro motion of an object
D.The buzzing sound produced by bees
Propagation of Sound
Sound Propagation and Waves:
A wave is a disturbance that travels through a medium, causing its particles to move and set nearby particles in motion.
The particles themselves do not move forward; rather, the disturbance moves forward.
Sound can be thought of as a wave because it is the movement of particles in a medium.
Sound waves are known as mechanical waves because they depend on the movement of particles.
Sound can be seen as variations in density or pressure in the medium.
Sound Propagation in Air:
Air is the most common medium for sound transmission.
When a vibrating object moves forward, it compresses the air in front of it, creating a high-pressure area known as compression (C).
This compression then moves away from the vibrating object.
When the object moves backward, it creates a low-pressure area called rarefaction (R).
As the object oscillates quickly, it produces a series of compressions and rarefactions in the air, forming the sound wave.
Compression indicates high pressure, whereas rarefaction indicates low pressure.
Compression (C) & Rarefaction (R) of sound
Pressure relates to the number of particles in a given volume of the medium.
A higher density of particles results in more pressure, and a lower density results in less pressure.
The distance between two consecutive compressions or rarefactions is known as the wavelength, λ.
The time taken for one complete oscillation of the density or pressure is called the time period, T.
The speed (v), frequency (f), and wavelength (λ) of sound are connected by the equation: v = fλ.
The law of reflection of sound states that the angles of incidence and reflection are equal concerning the normal to the reflecting surface at the point of incidence, and all three lie in the same plane.
Sound Waves are Longitudinal Waves
A wave is a disturbance that travels through a medium, causing its particles to move and set nearby particles in motion. The individual particles do not move forward themselves; instead, the disturbance moves through the medium.
Sound travels through the medium by a series of compressions (C) and rarefactions (R). These areas of closely packed and spaced out particles create longitudinal waves.
In longitudinal waves, the particles of the medium move in the same direction as the wave is travelling. They oscillate back and forth around their resting position without changing their location.
Sound waves are defined by how the particles in the medium move and are classified as mechanical waves. Air is the most common medium for sound transmission.
How Sound Waves Work
When a vibrating object moves forward, it compresses the air in front, creating a high-pressure area known as a compression (C).
When the object moves backward, it creates a low-pressure area called a rarefaction (R). Compression refers to high pressure, while rarefaction refers to low pressure.
Pressure is linked to the number of particles in a given volume; a denser medium results in higher pressure, and a less dense medium results in lower pressure.
Transverse Waves
Another type of wave is called a transverse wave. In these waves, particles do not move in the same direction as the wave travels but rather move up and down around their average position.
This means that in transverse waves, the individual particles move at a right angle to the direction of wave travel.
An example of a transverse wave is the ripples created on the surface of water when a pebble is dropped into it.
Try yourself:In longitudinal waves, how do particles of the medium move in relation to the direction of wave propagation?
A.Perpendicular to the direction of wave propagation
B.Parallel to the direction of wave propagation
C.Back and forth around their position of rest
D.They remain stationary
Characteristics of a Sound Wave
We can describe a sound wave by its:
Frequency
Amplitude
Speed
Key Characteristics of Sound Waves
Sound waves can be described by their frequency, amplitude, and speed.
The density and pressure of the medium change with distance as the sound wave travels.
Compressions are areas of high density and pressure, while rarefactions are areas of low pressure.
Wavelength is the distance between two consecutive compressions or rarefactions, represented by λ (lambda) with the SI unit of metre.
Frequency represents the number of oscillations per unit time and is measured in hertz (Hz), usually represented by ν (Greek letter, nu).
The time period (T) is the time taken for one complete oscillation, and frequency and time period are inversely related.
The audible range of hearing for average human beings is in the frequency range of 20 Hz – 20 kHz.
Sound waves with frequencies below the audible range are called “infrasonic,” and those above are called “ultrasonic.”
Pitch, Amplitude, and Loudness
Pitch is determined by the frequency of the sound wave, where higher frequency corresponds to a higher pitch.
Amplitude refers to the size of the maximum disturbance in the medium.
Loudness is a response of the ear to the intensity of sound, with greater amplitude producing a louder sound.
The loudness of a sound decreases as it travels further from its source.
Quality and Speed of Sound
Quality or timbre refers to the feature that distinguishes one sound from another with the same pitch and loudness.
Sound waves with a single frequency are called tones, while those with a mix of frequencies are called notes.
The speed of sound is the distance travelled by a point on a wave per unit time, calculated as Speed of sound = wavelength × frequency.
The speed of sound depends mainly on the nature and the temperature of the transmitting medium.
Intensity of Sound
Intensity of sound refers to the amount of sound energy passing through a unit area per second.
Loudness is a response of the ear to the intensity of sound.
Even sounds with the same intensity can be heard as different loudness due to differences in the ear’s sensitivity.
Speed of Sound In Different Media
Sound travels through a medium at a finite speed, which is slower than the speed of light.
The speed of sound depends on the properties of the medium and is affected by temperature; as temperature rises, the speed of sound in the medium increases.
The speed of sound varies in different media at a given temperature and decreases when moving from a solid to a gas.
Raising the temperature in a medium generally increases the speed of sound.
What is the definition of sound?
A.Sound is a type of energy that makes us see things through our eyes.
B.Sound is a form of energy that makes us hear things through our ears.
C.Sound is a type of energy that makes us taste things with our tongues.
D.Sound is a form of energy that makes us feel things with our hands.
Reflection of Sound
Sound waves behave like a rubber ball bouncing off a wall when they hit a solid or liquid surface.
Similar to light, sound follows the laws of reflection that you may have studied before.
When sound strikes a surface, it reflects in such a way that the angles of incidence and reflection are equal in relation to the normal (a line that is perpendicular to the surface) at the point where it hits.
These angles and the normal line are all in the same plane.
For sound waves to reflect, they need a sufficiently large obstacle, regardless of whether it is smooth or rough.
Echo
An echo is the sound we hear when the original sound is bounced back to us. The sensation of sound lingers in our brain for about 0.1 seconds.
Yelling or clapping near a suitable reflecting surface can create an echo.
Man producing echo
Conditions for Hearing a Distinct Echo
To hear a clear echo, there must be a time gap of at least 0.1 seconds between the original sound and the reflected sound. If we consider the speed of sound to be 344 m/s at a temperature of 22 °C in air, the total distance the sound travels must be at least (344 m/s) × 0.1 s = 34.4 m.
For a distinct echo, the minimum distance between the sound source and the reflecting surface should be half of the total distance, which is 17.2 metres. This distance can change depending on the temperature of the air.
Echoes can happen more than once because of repeated reflections.
Reverberation
When a sound is made in a large hall, it continues to exist because of multiple reflections from the walls until its intensity decreases enough that it can no longer be heard. This ongoing presence of sound due to reflections is known as reverberation.
Reverberation of Sound
Too much reverberation in an auditorium or large hall is not desirable.
To reduce reverberation, the walls and ceiling of the auditorium are usually covered with materials that absorb sound, such as compressed fibreboard, rough plaster, or curtains.
Additionally, the materials used for seating are selected for their sound-absorbing properties.
Question: A person clapped his hands near a cliff and heard the echo after 2 s. What is the distance of the cliff from the person if the speed of sound, v, is taken as 346 m/s?
Solution: Given,
Speed of sound: 346 m/s
Time taken for hearing the echo: 2 s
Distance travelled by the sound
Distance = 346 m/s × 2 s = 692 m
In 2 seconds, sound travels twice the distance between the cliff and the person.
Therefore, the distance between the cliff and the person is: Distance = 692 m / 2 = 346 m
Minimum Distance for Distinct Echoes
To hear distinct echoes, the minimum distance of the obstacle from the source of sound must be half of the total distance covered by the sound, which is at least 17.2 m.
This distance can change with temperature, as the speed of sound varies with temperature.
Uses of Multiple Reflections of Sound
Megaphones and Loudhailers: These devices are made to direct sound in a specific direction rather than spreading it everywhere. They usually have a tube and a conical opening that reflect sound waves, directing most of the sound towards the audience.
Stethoscopes: A stethoscope is a medical instrument used by doctors to listen to sounds inside the body, especially in the heart or lungs. The sound of the patient’s heartbeat reaches the doctor’s ears through multiple reflections of sound within the stethoscope.
Auditoriums and Concert Halls: In concert halls, conference rooms, and cinemas, the ceilings are often curved to make sure that sound reaches every corner of the hall. This helps to spread sound evenly throughout the space. The rolling of thunder is another example, caused by the repeated reflections of sound from different surfaces, like clouds and the ground.
Curved ceiling of conference hall
Try yourself:What is the minimum time interval required for a distinct echo to be heard?
A.0.01 seconds
B.0.1 seconds
C.1 second
D.10 seconds
Range of Hearing
The range of sound that humans can hear is from about 20 Hz to 20,000 Hz (where one Hz equals one cycle per second). Children under five years old and some animals, like dogs, can hear sounds up to 25 kHz (one kHz equals 1000 Hz).
As people age, their ears become less responsive to higher frequencies.
Sounds that are below 20 Hz are known as infrasonic sound or infrasound. If we were able to hear infrasound, we might perceive vibrations like those of a pendulum, similar to how we hear a bee’s wings.
Rhinoceroses communicate using infrasound at frequencies as low as 5 Hz. Animals like whales and elephants also produce sounds in the infrasound range.
It has been noted that some animals can detect low-frequency infrasound before earthquakes occur. Earthquakes generate low-frequency infrasound prior to the main shock waves, which may warn the animals.
Frequencies above 20 kHz are referred to as ultrasonic sound or ultrasound. Animals such as dolphins, bats, and porpoises produce ultrasound.
Certain moths have exceptionally sensitive hearing, enabling them to hear the high-frequency sounds made by bats, helping them to escape. Additionally, rats can create ultrasound during play.
Ultrasound has various applications in both medical and industrial fields.
Try yourself:What is the upper limit of the audible range for children under five years old and some animals?
A.10,000 Hz
B.15,000 Hz
C.20,000 Hz
D.25,000 Hz
Applications of Ultrasound
1. Cleaning Hard-to-Reach Objects
Objects are placed in a cleaning solution while ultrasonic waves are sent through it.
The high frequency of the waves causes dust, grease, and dirt to detach and fall away.
This method ensures thorough cleaning, even in complex shapes like spiral tubes or electronic components.
2. Detecting Cracks and Flaws
Ultrasound is used to identify cracks and flaws in metal blocks used in construction.
Ordinary sounds with longer wavelengths cannot detect these flaws as they bend around corners.
Ultrasonic waves pass through the metal block, and detectors identify the transmitted waves.
If there is a defect, the ultrasound reflects back, indicating the presence of a flaw.
3. Echocardiography
Ultrasonic waves are reflected from different parts of the heart to create an image.
Echocardiography is essential for diagnosing heart conditions and abnormalities.
4. Ultrasonography
Ultrasonic waves are used to image internal organs of the human body.
A doctor can examine organs like the liver, gall bladder, uterus, and kidneys.
Changes in tissue density cause the waves to reflect, turning into electrical signals.
These signals create images that help detect abnormalities, such as stones or tumours.
Ultrasonography is especially useful during pregnancy to check for congenital defects and growth issues.
5. Medical Treatment: Kidney Stone Breakage
Ultrasound can break small kidney stones into fine pieces.
The fragments can then be flushed out through urine, avoiding invasive procedures.
In earlier chapters, we learned about motion, its causes, and gravitation. Now, we explore another key idea—work, and its close partners, energy and power.
Energy is what keeps both living beings and machines going. We use it for everything—from running, playing, and thinking to operating engines and machines. Animals, too, need energy to survive, move, and help in tasks like carrying loads or ploughing fields.
Whether it comes from food or fuels like petrol and diesel, energy is what makes work possible. In this chapter, we’ll see how work and energy are connected and how they shape the world around us.
What is Work?
In our daily lives, we often refer to “work” as activities that require physical or mental effort. However, the scientific definition of work might differ from our usual understanding.
For instance, if you push a rock and it doesn’t move, even if you feel tired, scientifically, no work has been done.
Scientific Conception of Work
In scientific terms, work is defined as applying force to an object that causes it to move. Work done is calculated by multiplying the force applied by the distance the object moves in the direction of that force. Here are a few examples to clarify this concept:
Pushing a pebble: When you push a pebble and it moves, you apply force, and the pebble is displaced. Work is done here.
Pulling a trolley: If a girl pulls a trolley and it moves, the force she applies and the trolley’s movement mean work is done.
Lifting a book: When you lift a book, your force moves it upwards, so work is accomplished.
For work to occur, two key conditions must be satisfied:
There must be an application of force on the object.
The object must be displaced in the direction of the force.
It’s important to note that if there is no movement of the object, then the work done is zero. Additionally, an object capable of doing work is said to have energy.
Mathematically, the work done is calculated as:
Work done = force x displacement
where:
F is the constant force applied.
S is the displacement in the direction of the force.
The SI unit of work is the joule (J or Nm). Work has magnitude but no direction.
In summary, work occurs when a force makes an object move in that direction, and it is measured by the product of force and distance moved.
Work is a fundamental concept linked to energy, which exists in different forms such as kinetic energy and potential energy. Understanding work is essential for learning about energy conservation and transformation.
Try yourself:What is the scientific definition of work?
A.the product of force and displacement in the direction of the force
B.Work is the physical or mental effort involved in an activity.
C.Work is the conversion and transfer of energy in various systems.
D.Work is the product of force and displacement, regardless of the direction.
Work Done By A Constant Force
Work done on an object is defined as the amount of force multiplied by the distance the object moves in the direction of the applied force.
Work has only magnitude and no direction.
The unit of work is joule: 1 Joule (J) = 1 Newton × 1 metre (N·m)
Here, the unit of work is Newton metre (Nm) or joule (J).
Thus, 1 J is the amount of work done on an object when a force of 1 N displaces it by 1 m along the direction of the force.
Work done on an object by a force would be zero if the displacement of the object is zero.
Example 1
A force of 10 Newtons is applied to an object, causing it to be displaced by 5 meters. What is the work done on the object?
We can use the formula: W = F x S Force (F) = 10 Newtons Displacement (S) = 5 meters Putting these values into the equation, we have: W = (10 N) x (5 m) = 50 Joules
Therefore, the work done on the object is 50 Joules.
Example 2
A porter lifts a load of 15 kg from the ground and puts it on his head 1.5 m above the ground. Calculate the work done by him on the luggage.
Mass of luggage, m = 15 kg
Displacement, s = 1.5 m
Work done, W = F × s = mg × s = 15 kg × 10 m/s² × 1.5 m = 225 kg m/s² m = 225 Nm = 225 J
Work done is 225 J.
Force at an Angle
When a force is applied at an angle to the direction of displacement, only a part of the force causes motion. The formula to calculate work in such cases is: Work = Force × Distance × cos(θ).
Where:
Force is the magnitude of the constant force applied.
Distance is the displacement of the object in the direction of the force.
θ is the angle between the force and the displacement.
If the force and displacement are in the same direction (θ = 0), the formula simplifies to: Work = Force × Distance. This means that work done by a constant force is equal to the product of the force applied and the distance over which the force acts.
Example 3
A box is pushed with a force of 50 N at an angle of 30° to the horizontal. If the box moves 10 m, calculate the work done.
Force (F) = 50 N
Angle (θ) = 30°
Distance (d) = 10 m
Formula: Work = F × d × cos(θ)
Step-by-step solution:
Answer: The work done is approximately 433 J (Joules).
Positive, Negative & Zero Work Done
Positive Work: Work is considered positive if the displacement of the object is along the direction of the force applied. Example: Work done by a man is taken as positive when he moves from the ground floor to the second floor of his house.
Negative Work: Work is taken as negative if the displacement of the object is in the direction opposite to the force applied. Example: Work done by the man is negative when he descends from the second floor of the house to the ground floor.
Positive and Negative Work Done
Zero Work Done: If the displacement of an object is in a direction perpendicular to the application of force, work done is zero despite the fact that force is acting and there is some displacement too. Example: Imagine pushing a lawn roller forward. While gravity pulls it downward, the roller moves horizontally. Since gravity acts perpendicular to the roller’s movement, it does no work. This is an example of zero work, where a force doesn’t contribute to the displacement.
The gravitational potential energy of an object of mass m raised through a height, h, from the Earth’s surface is given by mgh.
Try yourself:The work done on an object does not depend upon the
A.displacement
B.force applied
C.angle between force and displacement
D.initial velocity of the object
What is Energy?
An object that can do work is said to have energy. Therefore, the energy of an object is its ability to perform work. When an object does work, it loses energy, while the object that receives the work gains energy. This means energy is transferred from one object to another. The unit of energy is the same as that of work, which is the joule (J). One joule is the energy needed to do one joule of work. A larger unit, the kilojoule (kJ), is also used, where 1 kJ equals 1000 J.
An object with energy can apply force on another object.
The energy of an object is measured by its ability to do work, indicating that any object with energy can perform work.
Energy Transformation
Forms of Energy
Energy exists in various forms in nature, including:
Mechanical energy
Heat energy
Electrical energy
Light energy
Chemical energy
Nuclear energy
Mechanical energy can be divided into two types:
Kinetic energy
Potential energy
Potential and Kinetic Energy
Kinetic Energy
The kinetic energy of an object is the energy possessed by it by virtue of its state of motion. A speeding vehicle, a rolling stone, a flying aircraft, flowing water, blowing wind, and a running athlete possess kinetic energy.
For an object of mass m and having a speed v, the kinetic energy is given by:
F = ma
Also, W=Fs
From the third equation of motion, we know that
v2 – u2 = 2as
Rearranging the equation, we get s = v2 – u2/2a
Substituting equation for work done by a moving body,
Taking initial velocity as zero, we get
where:
Ek is the kinetic energy.
m is the mass of the object.
v is the velocity of the object
Note : When two identical bodies are in motion, the body with a higher velocity has more KE.
Potential Energy
An object gains energy when it is lifted to a higher position because work is done against the force of gravity. This energy is known as gravitational potential energy. Gravitational potential energy is defined as the work done to raise an object from the ground to a certain height against gravity.
Let’s consider an object with a mass m being lifted to a height h above the ground.
The minimum force required to lift the object is equal to its weight, which is mg(where g is the acceleration due to gravity).
The work done on the object to lift it against gravity is given by the formula:
Work Done (W) = Force × Displacement
W = mg × h = mgh
The energy gained by the object is equal to the work done on it, which is mgh units.
This energy is the potential energy (Ep) of the object.
Ep = mgh
Note: It’s important to note that the work done by gravity depends only on the difference in vertical heights between the initial and final positions of the object, not on the path taken to move the object. For example, if a block is raised from position A to position B by taking two different paths, as long as the vertical height AB is the same (h), the work done on the object is still mgh.
Potential Energy of an Object at Height
Let’s consider an object with a mass m being lifted to a height h above the ground.
The minimum force required to lift the object is equal to its weight, which is mg (where g is the acceleration due to gravity).
The work done on the object to lift it against gravity is given by the formula:
Work Done (W) = Force × Displacement
W = mg × h = mgh
Since work done on the object is equal to mgh, an energy equal to mgh units is gained by the object. This is the potential energy (Ep) of the object.
Potential Energy (Ep) = mgh
Note: The work done by gravity depends only on the difference in vertical heights between the initial and final positions of the object, not on the path taken to move the object. For example, if a block is raised from position A to position B by taking two different paths, as long as the vertical height AB is the same (h), the work done on the object is still mgh.
Example 5
Find the energy possessed by an object of mass 10 kg when it is at a height of 6 m above the ground. Given g = 9.8 m/s².
Mass (m) = 10 kg
Height (h) = 6 m
Acceleration due to gravity (g) = 9.8 m/s²
Using the formula for potential energy: Ep = mgh
Substituting the values, we have:
Potential Energy = 10 × 9.8 × 6 = 588 J
Therefore, the potential energy of the object is 588 Joules.
Example 6
An object of mass 12 kg is at a certain height above the ground. If the potential energy of the object is 480 J, find the height at which the object is with respect to the ground. Given g = 10 m/s².
Mass of the object, m = 12 kg, potential energy, E = 480 J.
Using the formula E = mgh:
480 J = 12 × 10 × h
Solving for h:
h = 480 J / 120 kg m/s² = 4 m
The object is at a height of 4 m.
Also read: NCERT Solutions: Work and Energy
Law of Conservation of Energy
According to the law of conservation (transformation) of energy, we can neither create nor destroy energy. Energy can only change from one form to another; it cannot be created or destroyed. The total energy before and after the change remains constant. The total mechanical energy of an object is the sum of its kinetic energy and potential energy.
For an object falling freely, the potential energy decreases as it falls and transforms into kinetic energy. This process does not break the law of conservation of energy; instead, it demonstrates it, since the total mechanical energy remains unchanged during the fall.
As the object continues to fall, its potential energy decreases while its kinetic energy increases. If v is the object’s velocity at a given moment, the kinetic energy is 1/2 mv². Just before hitting the ground, h = 0, and v is at its maximum. Therefore, the kinetic energy is highest and the potential energy is lowest just before the object reaches the ground. However, the sum of potential energy and kinetic energy remains the same at all points, illustrating the law of conservation of energy.
Potential Energy + Kinetic Energy = Constant
or
mgh + 1/2 mv² = constant
The law of conservation of energy applies in all scenarios and for all types of transformations.
Try yourself:Water stored in a dam possesses
A.no energy
B.electrical energy
C.kinetic energy
D.potential energy
The diagram below shows a pendulum, which consists of a mass (m) connected to a fixed pivot point via a string of length (L).
Positions of the Pendulum
At the highest point (A): Here, the pendulum is briefly at rest, and all its energy is potential energy (PE). The height (h) of the mass above the lowest point determines how much potential energy it has. The potential energy can be calculated using the formula: PE = m · g · h, where g is the acceleration due to gravity.
At the lowest point (B): As the pendulum swings down, its potential energy is changed into kinetic energy (KE). At this point, its height (h) is zero, meaning it has no potential energy. Here, all its energy is kinetic energy, and the pendulum is moving at its highest speed. The kinetic energy can be calculated using the formula: KE = ½ m · v², where v is the velocity of the mass.
At the highest point on the other side (C): As the pendulum swings upwards, its kinetic energy is transformed back into potential energy.
The total mechanical energy of the pendulum, which is the sum of kinetic energy and potential energy, stays the same throughout its motion. This shows the law of conservation of energy, which states that energy cannot be created or destroyed; it can only be changed from one form to another.
In essence, the energy changes in a pendulum illustrate that the total mechanical energy remains constant, confirming that the total energy before and after the change is unchanged.
Rate of Doing Work or Power
The rate at which work is done or energy is transferred is known as Power. Power indicates how quickly or slowly work is performed. The formula for calculating power is:
Power = Work done / Time taken
The SI unit of power is a watt (W). One watt is defined as the power of an agent that does work at the rate of 1 joule per second (1 W = 1 J/s).
We use larger units for energy transfer, such as kilowatts (kW): 1 kilowatt = 1 kW = 1000 watts.
Other common units of power include:
1 megawatt (MW) = 106 watts
1 horsepower (hp) = 746 watts
Average Power
Understanding average power is important because it helps us see how quickly work is done over time, even if that speed changes. You can calculate average power by dividing the total energy used by the total time taken. This results in a single number that shows the overall power, regardless of variations in the work rate.
Average Power = Total energy consumed (or total work done) / Total time
The power of a person can change over time, meaning they might work at different speeds during various intervals.
According to the law of conservation of energy, energy can only change forms; it cannot be created or destroyed. The total energy before and after any change always remains constant. Energy exists in various forms in nature, such as kinetic energy, potential energy, heat energy, and chemical energy. The total of kinetic and potential energies in an object is referred to as its mechanical energy.
Try yourself:A machine does 18000 Joules of work in 3 minutes. What is the average power of the machine in watts?
The force responsible for objects falling towards Earth, the Moon orbiting Earth, and planets orbiting the Sun. Isaac Newton identified this as the universal gravitational force.
Newton’s Insight: Newton hypothesised that the same force that causes an apple to fall also keeps the Moon in orbit around Earth. This force acts towards the centre, known as the centripetal force.
Centripetal Force
Centripetal force is what keeps objects moving in a circular path. It pulls objects towards the centre of the circle, helping them keep moving in that circular motion.
The Moon’s motion around Earth is due to the centripetal force provided by Earth’s gravitational attraction. Without this force, the Moon would move in a straight line.
Newton’s Third Law
The Earth attracts an apple, and the apple attracts the Earth with an equal force (Newton’s Third Law).
Due to the Earth’s significantly larger mass, its acceleration towards the apple is negligible, so we don’t observe the Earth moving towards the apple or the Moon.
Examples of Centripetal Force
Universal Law of Gravitation
According to Newton’s law of gravitation, the force of gravitational attraction between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
This means:
The gravitational force gets weaker as you go higher up.
It also changes on the Earth’s surface, becoming weaker from the poles to the equator.
Formula: If M and m are the masses of two objects separated by a distance d, the gravitational force of attraction between them is given by: F = G M m⁄d2
where G is a constant, known as the Universal constant of gravitation.
The universal constant of gravitation G is numerically equal to the force of attraction between two objects of unit mass each separated by unit distance.
The value of G is 6.673 x 10-11 N m2 kg-2. This value was determined by Henry Cavendish (1731 – 1810) using a sensitive balance.
G is called a universal constant because its value does not depend on the nature of the intervening medium or temperature, or other physical conditions
As the value of G is extremely small, the gravitational force between regular objects is so small that it cannot be detected.
However, the force of attraction acting on an object due to Earth, the force of attraction between Earth and the moon, and the force experienced by planets due to the gravitational attraction of the Sun can be easily felt and measured.
Try yourself:Which of the following statements is true according to the universal law of gravitation?
A.The force of attraction between two objects depends on their masses and the distance between them.
B.The value of the universal constant of gravitation depends on the nature of the intervening medium.
C.The gravitational force between ordinary terrestrial objects is easily detected and measured.
D.The value of the universal constant of gravitation is 9.8 m/s2.
Example 1: Suppose we have two objects: Object A with a mass of 5 kilograms and Object B with a mass of 10 kilograms. The distance between the centres of these objects is 2 meters. We’ll assume the gravitational constant, G, to be approximately 6.674 × 10-11 N m2/kg2.
Solution:
Using the Universal Law of Gravitation, we can calculate the gravitational force between these objects:
F = (G * (m1 * m2)) / r2
F = (6.674 × 10-11 N m2/kg2 * (5 kg * 10 kg)) / (2 m)2
F = (6.674 × 10-11 N m2/kg2 * 50 kg2) / 4 m2
F ≈ 8.3425×10 −10N
Therefore, the gravitational force between Object A and Object B is approximately 8.3425×10 −10 Newtons.
Importance Of The Universal Law Of Gravitation
The universal law of gravitation explains various phenomena that were once thought to be unrelated:
The force that keeps us grounded on Earth
The moon’s orbit around the Earth
This same force governs the planets’ movement around the Sun
The tides caused by the moon and the Sun
Free Fall or Gravity
The force that pulls objects toward the Earth is known as the force of gravity.
For an object with mass m located on or near the Earth, this force can be calculated using the formula: F = GMm/R², where G is the universal gravitational constant, M is the mass of the Earth, and R is the radius of the Earth.
The acceleration produced in a freely falling object on account of the force of gravity is known as the acceleration due to gravity. It is denoted by the symbol ‘g’.
Gravitation Formula
To calculate the Value of g
The acceleration due to gravity at Earth’s surface is given by the formula: g = GM/R². The average value of g on the surface of the Earth is about 9.8 m/s². To find g, we use these constants:
G = 6.7 × 10-11N m²/kg² (Gravitational Constant)
M = 6 × 10 24 kg (Mass of the Earth)
R = 6.4 × 106 m (Radius of the Earth)
Here’s how the value of g is calculated.
Calculation of acceleration due to gravity
The motion of Objects under the influence of the Gravitational Force of the Earth
The value of g varies from place to place. On the surface of the earth value of g is more at the poles than at the equator. The value of g also decreases as one moves farther from the Earth.
Free Fall Motion
When an object falls towards the Earth under the force of gravity alone, we say that the object is in free fall. A freely falling object experiences a constant acceleration of g (=9.8ms-2) during its downward motion.
However, if an object is projected vertically upward with a certain velocity, its velocity goes on decreasing due to gravity, till it comes to rest and then starts falling vertically downward under gravity.
To demonstrate the impact of air resistance on falling objects, try this activity:Activity: Drop a piece of paper and a stone from the same height at the same time. Check if both hit the ground together. You will notice that the paper takes longer to fall because of air resistance. In a vacuum, both would fall at the same speed.
The three equations of motion, viz, (i) v = u + at, (ii) s = ut + 1/2 at2, and (iii) v2 – u2 = 2as, are true for the motion of objects under gravity. For free fall, the value of acceleration a = g = 9.8ms-2.
If an object is just let fall from a height, then in that case u = 0 and a = +g = +9.8ms–2.
If an object is projected vertically upwards with an initial velocity u, then a = -g = -9.8ms-2 and the object will go to a maximum height h where its final velocity becomes zero (i.e. v = 0). In such a case
Examples for the Three Equations of Motion Under Gravity
1. Using v = u+at:
2. Using s = ut + 1/2at2
2. Usingv2−u2=2as:
Mass
The mass of an object is a measure of its inertia. The mass of an object is constant and does not change from place to place. Greater mass means greater inertia, resisting changes in motion.
Mass and Weight
Weight
The weight of an object is the force with which it is attracted towards the Earth. The weight W of an object of mass m will be W = mg. Weight is a force acting vertically downwards. It means that it is a vector.
As the weight of an object is a force, its SI unit is Newton (N). An object of mass m = 1 kg has thus a weight of W = 1 x 9.8 = 9.8 N.
At a given place weight of an object is directly proportional to its mass, i.e, (at a given place). For this reason, at a given place, we may use the weight of an object as a measure of its mass.
Weight of Object on the Moon
The mass of an object stays the same no matter where it is. This is important for understanding the difference between mass and weight. Weight is the force that pulls an object towards the Earth or the Moon.
The force of gravity due to the moon is 1/6th of the force of gravity due to Earth. Hence Due to this very reason weight of an object on the moon will be 1/6th of its weight on Earth.
Try yourself:Question: Which of the following statements about weight is true?
A.Weight is a measure of the amount of matter in an object.
B.Weight is the force of gravity acting on an object.
C.Weight is a constant property of an object and does not change.
D.Weight is the same as mass.
Thrust and Pressure
The normal force acting on a surface, due to the weight of an object placed on the surface, is called ‘thrust’. As thrust is a sort of force hence its SI unit is “a newton” (N).
Thrust
The thrust on unit surface area is called pressure. Pressure Thus, pressure on a given object is the normal force acting on its surface per unit surface area. SI unit of pressure is N m-2, but it is also called pascal and denoted by the symbol Pa. ∴ 1 pascal (1 Pa) = 1 N m-2
The same force acting on a smaller area exerts a larger pressure. It is due to this reason that a nail or a pin has a pointed tip, and knives have sharp edges.
Given force acting on a larger area exerts a smaller pressure. It is due to this reason that the foundations of houses are made broad, the base of dams is made broad, sleepers are laid below the railway line and so on.
Pressure in Fluids
Fluid is that state of matter which can flow. All liquids and gases are fluids.
Fluids have weight and exert pressure on the base and walls of their container.
Any pressure in a confined fluid is transmitted equally in all directions.
The SI unit of pressure is the pascal, abbreviated as Pa.
Also read: Short & Long Answer Questions- Gravitation
Buoyancy
When an object is placed in a fluid, it feels a force pushing it upwards, known as upthrust or buoyant force. All objects experience this force when submerged in a fluid.
The strength of the buoyant force depends on the fluid’s density, causing the object to rise when released.
Buoyancy
Try yourself:What is the SI unit of weight?
A.Newton (N)
B.Kilogram (kg)
C.Pascal (Pa)
D.Meter (m)
Why Objects Float Or Sink When Placed On The Surface Of Water?
The ability of an object to float or sink in water depends on its density compared to the water’s density and the buoyant force acting on it. Density measures how much mass is in a certain volume.
Floating and Sinking on Surface of Water
When an object is placed in water, it experiences two main forces: buoyancy and gravity.
Buoyancy is the upward force on an object in a fluid, like water. This force arises from the pressure difference on the object’s top and bottom. According to Archimedes’ principle, the upward buoyant force equals the weight of the water displaced by the object. The more water the object displaces, the greater the buoyant force. If the object’s weight exceeds the buoyant force, it will sink; if the buoyant force is greater, it will float.
The weight of an object is the result of its mass multiplied by the acceleration due to gravity. While weight can change depending on location, the mass remains constant.
In summary, whether an object floats or sinks in water depends on the comparison between its weight and the buoyant force exerted by the water. If the object’s weight is greater, it will sink. If the buoyant force is greater, it will float.
Archimedes’ Principle
A Greek scientist Archimedes found a principle about buoyant force, which is the reduction in weight of an object when it is placed in a fluid. He realised this after he saw water spill from a bathtub when he got in. He ran through the streets shouting “Eureka!”, which means “I have found it”.
Archimedes’ Principle
According to Archimedes’ principle, “when an object is fully or partially placed in a fluid, it feels an upward force equal to the weight of the fluid it displaces.”
This principle has many uses, like in designing ships and submarines. It is also the basis for lactometers, which check the purity of milk, and hydrometers, which measure the density of liquids.
Try yourself:Archimedes’ principle states that:
A.The buoyant force on an object is equal to the weight of the fluid displaced by the object
B.The buoyant force on an object is equal to the volume of the fluid displaced by the object
C.The buoyant force on an object is equal to the mass of the fluid displaced by the object
D.The buoyant force on an object is equal to the density of the fluid displaced by the object
Motion is described in terms of position, velocity, and acceleration. Motion can be uniform (consistent speed) or non-uniform (changing speed).Historical Perspective:
Ancient Belief: Objects at rest are in their “natural state.”
Galileo & Newton: Challenged the old belief, developing new understanding of motion and its causes.
A force is an effort that changes the state of an object at rest or at motion. It can change an object’s direction and velocity. Force can also change the shape of an object.
It is the force that enables us to do any work. To do anything, either we pull or push the object. Therefore, pull or push is called force. Example: To open a door, either we push or pull it. A drawer is pulled to open and pushed to close.
Push or Pull is called Force
Effects of Force
Force can make a stationary body move.
Force can stop a moving body.
Force can change the direction of a moving object.
Effects of Force
Force can change the speed of a moving body.
Force can change the shape and size of an object.
Try yourself:Which of the following statements best defines force?
A.Force is the effort that changes the state of an object at rest or in motion.
B.Force is the energy that enables us to do any work.
C.Force is the push or pull applied to an object.
D.Force is the ability to change the direction and velocity of an object.
Balanced and Unbalanced Forces
In physics, forces can be classified as balanced or unbalanced based on their effects on an object’s motion. Mentioned below are the details of both these forces:
Balanced and Unbalanced Forces
Balanced Forces
If the resultant of applied forces is equal to zero, it is called balanced forces.
Balanced Force
Example: In the tug of war if both teams apply similar magnitude of forces in opposite directions, the rope does not move in either side. This happens because of balanced forces in which the resultant of applied forces becomes zero.
Balanced forces do not cause any change in the state of an object. Balanced forces are equal in magnitude and opposite in direction.
Balanced forces can change the shape and size of an object. Example: When forces are applied from both sides over a balloon, the size and shape of the balloon are changed.
Unbalanced Forces
If the resultant of applied forces are greater than zero, the forces are called unbalanced forces. An object at rest can be moved because of applying unbalanced forces.
Unbalanced Force
Unbalanced forces can do the following:
Move a stationary object.
Increase the speed of a moving object.
Decrease the speed of a moving object.
Stop a moving object.
Change the shape and size of an object.
Newton’s Laws of Motion
Newton studied the ideas of Galileo and gave the three laws of motion. These laws are known as Newton’s laws of motion: (i) Newton’s First Law of Motion (Law of Inertia). (ii) Newton’s Second Law of Motion. (iii) Newton’s Third Law of Motion.
First Law of Motion (Law of Inertia)
Any object remains in the state of rest or uniform motion along a straight line until it is compelled to change the state by applying an external force.
Illustration of Newton’s First Law of Motion
Explanation
If any object is in the state of rest, then it will remain in rest until an external force is applied to change its state.
Similarly, an object will remain in motion until an external force is applied over it to change its state.
This means all objects resist changing their state. The state of any object can be changed by applying external forces only.
Examples of Newton’s First Law of Motion in Everyday Life
A person standing inside a bus falls backwards when the bus starts moving suddenly. Explanation: This happens because the person and bus both are at rest while the bus is not moving, but as the bus starts moving, the legs of the person start moving along with the bus, but the rest portion of his body has the tendency to remain in rest. Because of this, the person falls backwards; if he is not alert.
A person standing inside a moving bus falls forward if the driver applies brakes suddenly. Explanation: This happens because when the bus is moving, the person standing in it is also in motion along with the bus. But when the driver applies brakes the speed of the bus decreases suddenly or the bus comes into a state of rest suddenly, in this condition the legs of the person, which are in contact with the bus come to rest while the rest part of his body tends to remain in motion. Because of this person falls forward if he is not alert.
Before hanging the wet clothes over the laundry line, usually, many jerks are given to the clothes to get them dried quickly. Because of jerks, droplets of water from the pores of the cloth fall on the ground, and a reduced amount of water in clothes dries them quickly. Explanation: This happens because when suddenly clothes are made in motion by giving jerks, the water droplets in them have the tendency to remain at rest and they are separated from the clothes and fall on the ground.
When a striker hits the pile of coins on the carom-board, the coin only at the bottom moves away leaving the rest of the pile of the coin in the same place. Explanation: This happens because when the pile is struck with a striker, the coin at the bottom comes in motion while the rest of the coin in the pile has the tendency to remain in the rest and they vertically fall on the carom-board and remain at the same place.
Galileo’s Idea of Motion
Galileo first said that objects move at a constant speed when no forces act on them.
Galileo’s Idea of Motion
This means if an object is moving on a frictionless path and no other force is acting upon it, the object will be moving forever. That is, there is no unbalanced force working on the object.
But practically it is not possible for any object. Because to attain the condition of zero, the unbalanced force is impossible.
Force of friction, Force of air, and many other forces are always acting upon an object.
Inertia and Mass
The property of an object because of which it resists getting disturbed in its state is called inertia. In other words, the natural tendency of an object that resist the change in the state of motion or rest of the object is called inertia.
The inertia of an object is measured by its mass. Inertia is directly proportional to the mass of the object.
Inertia increases with an increase in mass and decreases with a decrease in mass.
A heavy object will have more inertia than the lighter one. Since a heavy object has more inertia, thus it is more difficult to push or pull a heavy box over the ground than the lighter one.
Inertia is the natural tendency of an object to resist a change in its state of motion or of rest. The mass of an object is a measure of its inertia.
Try yourself:
Which of the following statements best describes a balanced force?
A.A force that can change the shape and size of an object.
B.A force that can stop a moving object.
C.A force that can move a stationary object.
D.A force that does not cause any change in the state of an object.
Second Law of Motion
The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
Illustration of Force depending on Mass and Acceleration
(a) Momentum
Momentum is the power of motion of an object.
The product of velocity and mass is called momentum. Momentum is denoted by ‘p’.
Therefore, Momentum of the object = Mass × Velocity (p = m × v), where p = momentum, m = mass of the object and v = velocity of the object.
Some explanations to understand the momentum:
A person gets injured in the case of hitting a moving object, such as a stone, pebbles, or anything because of the momentum of the object.
Even a small bullet can kill a person when it is fired from a gun because of its momentum due to great velocity.
A person gets injured severely when hit by a moving vehicle because of the momentum of the vehicle due to mass and velocity.
(b) Momentum and Mass
Since momentum is the product of mass and velocity (p = m × v) of an object. This means momentum is directly proportional to mass and velocity. Momentum increases with the increase of either the mass or velocity of an object.
This means if a lighter and a heavier object is moving with the same velocity, then the heavier object will have more momentum than the lighter one.
If a small object is moving with great velocity, it has tremendous momentum. And because of momentum, it can harm an object more severely.Example: A small bullet having a small mass even kills a person when it is fired from a gun.
Usually, road accidents prove more fatal because of high speed than slower speed. This happens because vehicles running at high speed have greater momentum compared to a vehicles running at a slower speed.
(c) Unit of Momentum
We know that, Momentum (p) = m × v
SI unit of mass = kg
SI unit of velocity = m/s
Therefore,
p = kg × m/s ⇒ p = kgm/s
The momentum of an object which is in the state of rest: Let, an object with mass ‘m’ be in the rest. Since, the object is at rest, therefore, its velocity, v = 0 ∵ Momentum = mass × velocity ⇒ p = m × 0 = 0 Thus, the momentum of an object in the rest, i.e. non-moving is equal to zero.
Numerical Problems Based on Momentum
Example 1:What will be the momentum of a stone that has a mass of 10 kg when it is thrown with a velocity of 2 m/s? Solution: Mass (m) = 10 kg, Velocity (v) = 2 m/s. ∵ Momentum (p) = Mass (m) × Velocity (v) ⇒ p = 10 kg × 2 m/s = 20 kg m/s. Thus, the momentum of the stone = 20 kg m/s.
Example 2:Calculate the momentum of a bullet of 25 g when it is fired from a gun with a velocity of 100 m/s. Solution: Given the velocity of the bullet (v) = 100 m/s, Mass of the bullet (m) = 25 g = 25/1000 kg = 0.025 kg. ∵ p = m × v ⇒ p = 0.025 × 100 = 2.5 kg m/s. Thus, the Momentum of the bullet = 2.5 kg m/s.
Example 3:Calculate the momentum of a bullet having a mass of 25 g is thrown using a hand with a velocity of 0.1 m/s. Solution: Given the velocity of the bullet (v) = 0.1 m/s, Mass of the bullet (m) = 25 g = 25/1000 kg = 0.025 kg. ∵ Momentum (p) = Mass (m) × Velocity (v) ⇒ p = 0.025 kg × 0.1 = 0.0025 kg m/s Thus, Momentum of the bullet = 0.0025 kg m/s.
Example 4:The mass of a goods lorry is 4000 kg and the mass of goods loaded on it is 20000 kg. If the lorry is moving with a velocity of 2 m/s, what will be its momentum? Solution: Velocity (v) = 2 m/s ⇒ Mass of lorry = 4000 kg ⇒ Mass of goods on the lorry = 20000 kg. ⇒ Total mass (m) on the lorry = 4000 kg + 20000 kg = 24000 kg ∵ Momentum (p) = Mass (m) × Velocity (v) ⇒ p = 24000 kg × 2 = 48000 kg m/s Thus, the Momentum of the lorry = 48000 kg m/s.
Example 5: A car having a mass of 1000 kg is moving with a velocity of 0.5 m/s. What will be its momentum? Solution: Velocity of the car (v) = 0.5 m/s ⇒ Mass of the car (m) = 1000 kg. ∵ Momentum (p) = Mass (m) × Velocity (v) ⇒ p = 1000 kg × 0.5 m/s = 500 kg m/s Thus, the Momentum of the car = 500 kg m/s.
Mathematical Formulation of the Second Law of Motion
Suppose the mass of an object = m kg Initial velocity of an object = u m/s, The final velocity of an object = v m/s. Initial momentum, p1 = mu, Final momentum, p2 = mv. Change in momentum = Final momentum – Initial momentum Change in momentum = mv – mu = m(v – u) Rate of change of momentum = Change in momentum/Time taken Rate of change of momentum = m(v-u)/t
According to 2nd law, this rate of change is momentum is directly proportional to force, i.e.
Newton’s Second Law of MotionWe know that: a = (v-u)/t (From 1st equation of motion) ⇒ F = kma, where k is a constant. Its value can be assumed as 1. ⇒ F = 1 × m × a = ma
SI unit = kgms-2 or Newton
1 Newton: When an acceleration of 1 m/s2 is seen in a body of mass 1 kg, then the force applied on the body is said to be 1 Newton.
Proof of Newton’s First Law of Motion from the Second Law The first law states that if external force F = 0, then a moving body keeps moving with the same velocity, or a body at rest continues to be at rest. ⇒ F = 0 We know, F = m(v-u)/t
A body is moving with initial velocity u then, m(v-u)/t = 0 ⇒ v – u = 0 ⇒ v = u Thus, the final velocity is also the same.
A body is at rest i.e., u = 0 ⇒ u = v = 0 Hence, the body will continue to be at rest.
Try yourself:Which of the following statements is true about momentum?
A.Momentum is the product of mass and acceleration.
B.Momentum is directly proportional to mass of the object.
C.Momentum is the product of force and time.
D.Momentum is the rate of change of velocity of an object.
Third Law of Motion
According to Newton’s Third Law of Motion, for every action, there is an equal and opposite reaction.
Newton’s Third Law of Motion explains the interaction between two objects when a force is applied.
Action and Reaction Forces: – When one object exerts a force on another, the second object exerts an equal and opposite force back on the first. – These forces are equal in magnitude but opposite in direction. – These forces act on different objects, never on the same object.
Example: Gun Recoil:– When a gun is fired, it exerts a forward force on the bullet. – The bullet exerts an equal and opposite force on the gun, causing it to recoil. – The gun’s greater mass results in less acceleration compared to the bullet.
In our daily lives, objects such as birds, fish, and cars can be either at rest or in motion. Motion is observed when an object’s position changes over time. Sometimes, motion is inferred indirectly, like noticing dust moving to deduce air movement. Perception of motion can vary: passengers in a moving bus see trees moving backward, while onlookers outside the bus see both the bus and its passengers in motion.
Motion
Describing Motion
To describe an object’s motion, we use a reference point, or origin, as a fixed location. For instance, if a school is 2 km north of a railway station, the railway station is the reference point. This origin helps us measure and describe the object’s position relative to it.
What is Motion?
A body is said to be in a state of motion when its position changes continuously with reference to a point. Motion can be of different types depending upon the type of path by which the object is travelling through:
Types of Motion
Circulatory motion/Circular motion: In a circular path.
Linear motion: In a straight-line path.
Oscillatory/Vibratory motion: To and fro path with respect to origin.
Motion Along a Straight Line
The simplest type of motion is along a straight line. Let’s understand this with an example. Imagine an object moving along a straight path, starting from point O, which we use as the reference point.
Motion along a straight line
Example Description
Initial Motion: The object starts at point O and moves to point A via points C and B.
Return Motion: The object then travels back from A to C through B.
Distance Covered
The total path length covered by the object is the sum of the distances traveled:
Distance from O to A: 60 km
Distance from A to C: 35 km
Total Distance from O to C = OA + AC = 60 km + 35 km = 95 km
Distance is a scalar quantity, meaning it only has magnitude and no direction.
Scalar quantity: It is the physical quantity having its own magnitude but no direction. Example: Distance, Speed.
Vector quantity: It is the physical quantity that requires both magnitude and direction. Example: Displacement, Velocity.
Distance and Displacement
1. Distance
The actual path of length travelled by an object during its journey from its initial position to its final position is called the distance.
Distance is a scalar quantity that requires only magnitude but no direction to explain it.
Example: Ramesh travelled 65 km. (Distance is measured by odometer in vehicles.)
2. Displacement
The shortest distance travelled by an object during its journey from its initial position to its final position is called displacement.
Displacement is a vector quantity requiring both magnitude and direction for its explanation.
Example: Ramesh travelled 65 km southwest from Clock Tower.
Displacement can be zero (when the initial point and final point of motion are the same) Example: Circular motion.
Example of Zero Displacement
Try yourself:What is the definition of distance?
A.A body is said to be in a state of rest when its position does not change with respect to a reference point.
B.A body is said to be in a state of motion when its position changes continuously with reference to a point.
C.The actual path of length travelled by an object during its journey from its initial position to its final position.
D.The physical quantity that requires both magnitude and direction.
Example 1: A body travels in a semicircular path of radius 10 m starting its motion from point ‘A’ to point ‘B’. Calculate the distance and displacement.
Sol. Given, π = 3.14, R = 10 m
The distance is the length of the semicircular path.
Distance = Circumference of a circle ÷ 2
Distance = 2πR ÷ 2
Distance = πR = 3.14 × 10 = 31.4 m
Where as ,
Displacement = 2 × R = 2 × 10 = 20 m
Example 2: A body travels 4 km towards North then he turns to his right and travels another 4 km before coming to rest. Calculate
(i) total distance travelled,
(ii) total displacement.
Sol. The total distance is the sum of all the paths travelled:
Total Distance = 4km (North) + 4km (Right) = 8km
Since displacement is the shortest straight-line distance between the starting point and the final point.
The path forms a right triangle, where:
One leg = 4 km (North direction),
Other leg = 4 km (Right direction).
Try yourself:What is the difference between distance and displacement?
A.Distance is a vector quantity, while displacement is a scalar quantity.
B.Distance requires both magnitude and direction, while displacement only requires magnitude.
C.Distance is measured in kilometers, while displacement is measured in meters.
D.Distance is the actual path traveled by an object, while displacement is the shortest distance between the initial and final positions.
Uniform and Non-uniform Motion
1. Uniform Motion
When a body travels equal distances in equal intervals of time, then the motion is said to be a uniform motion.
2. Non-uniform Motion
In this type of motion, the body travels unequal distances in equal intervals of time.
Two types of non-uniform motion: (i) Accelerated Motion: When the motion of a body increases with time. (ii) De-accelerated Motion: When the motion of a body decreases with time.
Measuring the Rate of Motion
The measurement of distance travelled by a body per unit of time is called speed. i.e. Speed (v) = Distance Travelled/Time Taken = s/t
SI unit:m/s (meters/second)
If a body is executing uniform motion, then there will be a constant speed.
If a body is travelling with a non-uniform motion, then the speed will not remain uniform but have different values throughout the motion of such a body.
For non-uniform motion, the average speed will describe one single value of speed throughout the motion of the body. i.e. Average speed = Total distance travelled/Total time taken
Conversion Factor Change from km/hr to m/s = 1000m/(60×60)s = 5/18 m/s
Example: What will be the speed of body in m/s and km/hr if it travels 40 km in 5 hrs? Sol: Distance (s) = 40 km Time (t) = 5 hrs. Speed (in km/hr) = Total distance/Total time = 40/5 = 8 km/hr 40 km = 40 × 1000 m = 40,000 m 5 hrs = 5 × 60 × 60 sec. Speed (in m/s) = (40 × 1000)/(5×60 ×60) = 80/36 = 2.22 m/s
Speed with Direction
It is the speed of a body in a given direction.
The measurement of displacement travelled by a body per unit of time is called velocity. i.e.Velocity = Displacement/Time
SI unit of velocity: ms-1
Velocity is a vector quantity. Its value changes when either its magnitude or direction changes.
It can be positive (+ve), negative (-ve) or zero.
For non-uniform motion in a given line, average velocity will be calculated in the same way as done in average speed. i.e.Average velocity = Total displacement/Total time
For uniformly changing velocity, the average velocity can be calculated as follows:-
Avg. Velocity (vavg) = (Initial velocity + Final velocity)/2 = (u+v)/2 where, u = initial velocity, v = final velocity
Example 1: During the first half of a journey by a body it travels with a speed of 40 km/hr and in the next half it travels at a speed of 20 km/hr. Calculate the average speed of the whole journey.
Sol: The average speed for a journey where the distances are equal but the speeds are different is not simply the arithmetic mean
When a body covers equal distances at different speeds, the correct formula for average speed is: Given:
Speed during the first half (v1) = 40 km/hr
Speed during the second half (v2) = 20 km/hr
Now, use the correct formula:
Example 2: A car travels 20 km in first hour, 40 km in second hour and 30 km in third hour. Calculate the average speed of the vehicle.
Sol: Speed in 1st hour = 20 km/hr
Distance travelled during 1st hr = 1 × 20= 20 km
Speed in 2nd hour = 40 km/hr
Distance travelled during 2nd hr = 1 × 40= 40 km
Speed in 3rd hour = 30 km/hr
Distance travelled during 3rd hr = 1 × 30= 30 km
Average speed = Total distance travelled/Total time taken
= (20 + 40 + 30)/3 = 90/3 = 30 km/hr
Try yourself:What is the definition of uniform motion?
A.When a body travels unequal distances in equal intervals of time.
B.When a body travels equal distances in equal intervals of time.
C.When the motion of a body increases with time.
D.When the motion of a body decreases with time.
Rate of Change of Velocity
Acceleration is seen in non-uniform motion and it can be defined as the rate of change of velocity with time. i.e. Acceleration (a) = Change in velocity/Time = (v-u)/t where, v = final velocity, u = initial velocity
Here, v > u, then ‘a’ will be positive (+ve). If v is greater than u, then acceleration (a) will be positive.
Example: A car speed increases from 40 km/hr to 60 km/hr in 5 sec. Calculate the acceleration of car. Sol. u = 40km/hr = (40×5)/18 = 100/9 = 11.11 m/s v = 60 km/hr = (60×5)/18 = 150/9 = 16.66 m/s t = 5 sec a = (v-u)/t = (16.66 – 11.11)/5 = 5.55/5 = 1.11 ms-2
Try yourself:Which quantity requires both magnitude and direction.
A.Distance
B.Displacement
C.Speed
D.None of these
Retardation/Deceleration
Deceleration is seen in non-uniform motion during decrease in velocity with time. It has same definition as acceleration. i.e. Deceleration (a’) = Change in velocity/Time = (v-u)/t
Here, v < u, ‘a’ = negative (-ve).
Example: A car travelling with a speed of 20 km/hr comes into rest in 0.5 hrs. What will be the value of its retardation? Sol. v = 0 km/hr, u = 20 km/hr, t = 0.5 hrs Retardation, a = (v-u)/t = (0-20)/0.5 = -200/5 = -40 km hr-2
Graphical Representation of Motion
1. Distance-Time Graph (s/t graph)
(i)s/t graph for uniform motion: (ii)s/t graph for non-uniform motion: (iii) s/t graph for a body at rest: v = (s2 – s1)/(t2 – t1) But, s2 – s1 ∴ v = 0/(t2 – t1) or v = 0
2. Velocity-Time Graph (v/t graph)
(i) v/t graph for uniform motion: a = (v2 – v1)/(t2 – t1) But, v2 – v1 ∴ a = 0/(t2 – t1) or a = 0 (ii) v/t graph for uniformly accelerated motion:
In uniformly accelerated motion, there will be an equal increase in velocity in equal intervals of time throughout the motion of the body. (iii) v/t graph for non-uniformly accelerated motion: a2 ≠ a1 (iv) v/t graph for uniformly decelerated motion: a1‘ = a2‘ (v) v/t graph for non-uniformly decelerated motion:
Note: In v/t graph, the area enclosed between any two time intervals, t2 – t1, will represent the total displacement by that body.
The displacement can also be calculated as the area of the trapezium formed by the v/t graph:
= Area of ∆ABC + Area of rectangle ACDB = ½ × (v2 – v1)×(t2 – t1) + v1× (t2 – t1)
Example: From the information given in the s/t graph, which of the following body ‘A’ or ‘B’ will be faster? Sol. vA > vB
Try yourself:Which of the following statements is true about acceleration?
A.Acceleration is only seen in uniform motion.
B.Acceleration is the rate of change of velocity with time.
C.Acceleration is always negative.
D.Acceleration is the rate of change of distance with time.
Equations of Motion by Graphical Method
Also read: NCERT Solutions: Motion
1. First Equation: v = u + at
Final velocity = Initial velocity + Acceleration × Time Graphical Derivation Suppose a body has initial velocity ‘u’ (i.e., velocity at time t = 0 sec.) at point ‘A’ and this velocity changes to ‘v’ at point ‘B’ in ‘t’ secs. i.e., final velocity will be ‘v’.
For such a body there will be an acceleration. a = Change in velocity/Change in Time ⇒ a = (OB – OA)/(OC-0) = (v-u)/(t-0) ⇒ a = (v-u)/t ⇒ v = u + at
2. Second Equation: s = ut + ½ at2
Distance travelled by object = Area of OABC (trapezium) = Area of OADC (rectangle) + Area of ∆ABD = OA × AD + ½ × AD × BD = u × t + ½ × t × (v – u) = ut + ½ × t × at ⇒ s = ut + ½ at2 (∵a = (v-u)/t)
3. Third Equation: v2 = u2 + 2as
s = Area of trapezium OABC
Example 1: A car starting from rest moves with a uniform acceleration of 0.1 ms-2 for 4 mins. Find the speed and distance travelled. Sol: u = 0 ms-1 (∵ car is at rest), a = 0.1 ms-2, t = 4 × 60 = 240 sec. v = ? From, v = u + at v = 0 + (0.1 × 240) = 24 ms-1
Example 2: The brakes applied to a car produces a deceleration of 6 ms -2 in the opposite direction to the motion. If a car requires 2 sec. to stop after the application of brakes, calculate the distance travelled by the car during this time. Sol: Deceleration, a = − 6 ms-2; Time, t = 2 sec. Distance, s =? Final velocity, v = 0 ms-1 (∵ car comes to rest) Now, v = u + at ⇒ u = v – at = 0 – (-6×2) = 12 ms-1 s = ut + ½ at2 = 12 × 2 + ½ (-6 × 22) = 24 – 12 = 12 m
Try yourself:A car is moving with an initial velocity of 20 m/s. If it accelerates at a rate of 5 m/s? for 4 seconds, what is its final velocity?
A.15 m/s
B.40 m/s
C.35 m/s
D.45 m/s
Uniform Circular Motion
If a body is moving in a circular path with uniform speed, then it is said to be executing the uniform circular motion.
In such a motion the speed may be the same throughout the motion but its velocity (which is tangential) is different at each and every point of its motion. Thus, uniform circular motion is an accelerated motion.
Direction at different points while executing circular motion
Have you ever wondered how your body moves, breathes, or even thinks? All these activities are possible because our body is made up of tiny building blocks called cells. All living organisms are made up of cells, which are the basic units of structure and function in life.
In unicellular organisms like Amoeba, a single cell performs all essential functions such as movement, food intake, breathing, and waste removal.
In contrast, multicellular organisms have millions of cells, each specialised to do a particular job efficiently. For example, in humans, muscle cells help in movement, nerve cells carry messages, and blood cells transport substances like oxygen and food. In plants, vascular tissues like xylem and phloem help move water and food throughout the plant.
In multicellular organisms, cells with similar jobs group together to form tissues. A tissue is a group of cells that are similar in structure and work together to perform a specific function. Examples of tissues include blood, muscle, and phloem. Grouping cells into tissues helps the body work more efficiently, as each tissue takes care of a certain task.
Tissue
Are Plants and Animals Made of the Same Types of Tissues?
Plants and animals are made up of different types of tissues, each serving specific roles that match their way of life.
1. Structure and Movement:
Plants: Plants stay in one place and need a strong structure to support themselves. Their supportive tissues, which mainly consist of dead cells, provide rigidity.
Animals: Animals are mobile, moving around to find food, mates, and shelter. Their tissues are mostly living and specialised, allowing for movement and flexibility.
2. Growth Patterns:
Plants: Plants grow in specific areas called meristems, where new cells are formed continuously. Once certain parts mature, they stop growing.
Animals: In contrast, animals grow more evenly, without clear areas of active and inactive cell division.
3. Complexity and Specialisation:
Plants: Plant tissues are divided into two main categories: meristematic and permanent. Meristematic tissue is for growth, while permanent tissues include simple types (like parenchyma, collenchyma, and sclerenchyma) and complex types (such as xylem and phloem).
Animals: Animal tissues are classified into four main types: epithelial, connective, muscular, and nervous tissues. Epithelial tissue is further divided based on shape and function into squamous, cuboidal, columnar, ciliated, and glandular types. Connective tissues consist of areolar, adipose, bone, tendon, ligament, cartilage, and blood. Muscle tissues include striated, unstriated, and cardiac types. Nervous tissue is made up of neurons that transmit signals.
Plant Tissues
Plant tissues are classified into two main types: meristematic and permanent.
Meristematic Tissue: This is the dividing tissue present in the growing regions of the plant.
Permanent Tissues: These are derived from meristematic tissue once they lose the ability to divide. Permanent tissues are further classified into simple and complex tissues.
The three types of simple tissues are:
Parenchyma
Collenchyma
Sclerenchyma
The two types of complex tissues are:
Xylem
Phloem
1. Meristematic Tissue
Meristematic tissue is the growth tissue found in the areas of the plant that are still developing. The growth of plants happens in specific areas because of this special tissue that divides.
These cells are small, round, or polygonal with dense cytoplasm. They are very active and have thin cellulose walls and noticeable nuclei. They do not have vacuoles.
Meristematic tissues can be classified based on their location:
Apical Meristem: Found at the tips of stems and roots, this type increases the length of both.
Lateral Meristem (cambium): Located on the sides of stems and roots, it helps to thicken them.
Intercalary Meristem: Found near the nodes (the points on a stem where leaves or branches grow). It contributes to the length between two nodes.Intercalary meristem section
Longitudinal section of shoot apex showing location of meristem and young leaves
Try yourself:
Which type of meristematic tissue is responsible for increasing the length of stems and roots?
A.Apical Meristem
B.Lateral Meristem
C.Intercalary Meristem
D.None of the above
2. Permanent Tissues
Permanent tissues come from meristematic cells that can no longer divide. These cells change to take on specific roles. They can be either living or dead, and their walls can be thin or thick, with the thickening being either regular or irregular.
Permanent tissues are further divided into two main groups: Simple Permanent Tissue and Complex Permanent Tissue.
Simple Permanent Tissue
A few layers of cells beneath the epidermis are generally known as simple permanent tissue.
This type consists of one kind of cell that serves the same function. The three types of simple tissues are:
Parenchyma
Collenchyma
Sclerenchyma
Complex permanent tissues are made up of more than one type of cell and include:
Xylem
Phloem
Also read: Short & Long Answer Questions- Tissues
(a) Parenchyma
Parenchyma cells are thin-walled, living cells that make up the basic packing tissue of all plant parts. They can be oval, spherical, or polygonal in shape, and are loosely arranged with small and large spaces in between. Their main role is to store food.
When parenchyma contains chlorophyll, it is called chlorenchyma, which is important for photosynthesis.
In aquatic plants, parenchyma cells have large air spaces that help store gases and keep the plants afloat. This special type of parenchyma is known as aerenchyma.
Parenchyma Tissue in Transverse and Longitudinal section
(b) Collenchyma
Collenchyma cells are living cells with thickened corners. The uneven thickness of their walls gives mechanical support and flexibility to the plant, allowing it to bend without breaking.
(c) Sclerenchyma
Sclerenchyma cells are usually dead, thick, and have tough walls. They have a narrow space inside. There are two types of sclerenchyma:
Fibres, which are long and spindle-shaped with pointed ends.
Sclereids, which are shorter and broader cells, sometimes called stone cells. This type of tissue provides strong support and helps plants withstand various stresses.Sclerenchyma Tissue
(d) Epidermis
The epidermis is the outer protective layer of plant parts that stops pathogens and pests from entering.
Epidermal cells are elongated and tightly packed, generally having thick outer and side walls, while the inner walls are thinner.
The thick outer walls contain a fatty substance called cutin, which makes them waterproof.
In some plants, like those in deserts, the epidermis has a thick waxy coating of cutin on its outer surface.
Root epidermal cells, which help absorb water, often have long hair-like extensions that increase the area available for absorption.
Typically, the epidermis is a single layer but has small openings called stomata that allow gas exchange. These are surrounded by two guard cells.Epidermis Water loss in the form of vapour, known as transpiration, also occurs through stomata. As plants age, the outer protective layer changes. A strip of secondary meristem in the cortex forms layers of cells called cork. Cork cells are dead, tightly packed without spaces, and contain a substance called suberin, making them impermeable to gases and water.
Epidermal cells of roots have long, hair-like structures called root hairs. These hairs help increase the surface area for better absorption of water and minerals from the soil.
On many plants, the outer surface has trichomes, which are hair-like extensions of the epidermis. These can be glandular or non-glandular and create a layer of still air on the surface, which provides insulation.
Cork is the outer protective layer found on older stems and roots. It arises from a type of lateral meristem known as cork cambium.
The cork cambium generates a secondary layer called phelloderm on the inside and cork or phellem on the outside.
Cork cells are dead and tightly packed, lacking intercellular spaces. Their walls contain a substance called suberin, which makes them resistant to gases and water.
Some cork cells have small openings called lenticels, which allow for gas exchange.
Older cork cells die and fill with substances like tannins, resins, and air.
Try yourself:
Which type of permanent tissue is responsible for providing mechanical support and flexibility to the plant?
A.Parenchyma
B.Collenchyma
C.Sclerenchyma
D.Epidermis
(ii) Complex Permanent Tissues
Complex permanent tissues consist of multiple cell types working together for a specific function.
These tissues are mainly divided into two types:
(a) Xylem
Xylem is a complex tissue that is crucial for transporting water and minerals from the soil to different parts of the plant through the roots. It includes tracheids, xylem vessels, xylem fibres, and xylem parenchyma.
Dead tubular tracheids and xylem vessels assist in moving water from roots to shoots. Living xylem parenchyma stores food.
Xylem Elements
Dead xylem fibres provide structural support. Vessels are long tubes formed by the end-to-end joining of many dead cells, with lignified walls that have pits.Working of Xylem and Phloem in plant
(b) Phloem
Phloem consists of five cell types: sieve cells, sieve tubes, companion cells, phloem fibres, and phloem parenchyma.
Sieve tubes are tubular cells with holes in their walls that transport food from the leaves to other parts of the plant. All phloem cells, except for phloem fibres, are living cells.
Phloem is the tissue responsible for transporting food that is made in leaves to other areas of the plant. It consists of five types of cells:
Sieve cells
Sieve tubes
Companion cells
Phloem fibers
Phloem parenchyma
Section of phloem
All the cells in the phloem are living, except for the phloem fibres. The main function of phloem is to transport food from the leaves to other parts of the plant, which is essential for the plant’s nutrition and growth.
Animal Tissues
Animal tissues are groups of cells that have similar structures and functions, working together to carry out specific tasks in the bodies of animals. These tissues can be divided into four main types based on their structure and function: epithelial tissue, connective tissue, muscular tissue, and nervous tissue.
Epithelial Tissues in Animals
Definition: Epithelial tissues are the protective coverings in the animal body. Functions:
Cover the majority of organs and cavities in the body.
Create barriers to separate different body systems.
Control the exchange of materials between the body and the outside environment.
Locations: Skin, lining of the mouth, lining of blood vessels, alveoli in lungs, kidney tubules.
Characteristics:
Cells are tightly packed, forming a continuous layer.
There is minimal cementing material between cells and almost no spaces.
All substances entering or leaving the body must pass through at least one layer of epithelium.
Epithelial tissue is classified by shape and function into several types, including cuboidal, columnar, ciliated, and glandular.
Try yourself:
Which type of tissue is responsible for transporting food from the leaves to other parts of the plant?
A.Xylem
B.Phloem
C.Epithelial
D.Connective
Connective Tissue
Connective tissues are identified by loosely spaced cells within a matrix that can be jelly-like, fluid, dense, or rigid, depending on the tissue’s purpose. Types of connective tissues in our body include:
Areolar tissue
Adipose tissue
Bone
Tendon
Ligament
Cartilage
Blood
Muscular Tissue
Definition: Muscular tissue is made up of long cells known as muscle fibres, which are responsible for movement.
Special Proteins: These tissues contain proteins that enable muscles to contract and relax, allowing movement.
Types of Muscular Tissue:
(i) Skeletal Muscles (Striated Muscles):
Function: Enables voluntary movements, like moving limbs.
Characteristics:
Attached to bones to facilitate body movement.
Under a microscope, these muscles display alternating light and dark bands (striations).
Cells are long, cylindrical, unbranched, and have multiple nuclei.
(ii) Smooth Muscles (Unstriated Muscles):
Function: Controls involuntary movements, such as food movement in the digestive tract and blood vessel contraction.
Locations: Found in the eye’s iris, ureters, bronchi of the lungs, and more.
Characteristics:
Cells are spindle-shaped and have a single nucleus.
These muscles do not have striations, hence called unstriated.
(iii) Cardiac Muscles:
Function: Regulates the rhythmic contraction and relaxation of the heart.
Characteristics:
Cells are cylindrical, branched, and have one nucleus.
These muscles operate involuntarily, without conscious control.
Voluntary vs. Involuntary Muscles
Voluntary Muscles: These muscles can be moved by our conscious will. They are found in our limbs and we can decide when to move them or stop.
Involuntary Muscles: These muscles work automatically without us having to think about it (for example, smooth and cardiac muscles). They are located in various parts of the body, such as the iris of the eye, ureters, and bronchi of the lungs.
Nervous Tissue
Nervous tissue plays a key role in fast communication within the body and is made up of neurons.
These cells are highly specialised to be stimulated and quickly send messages from one part of the body to another.
It includes the brain, spinal cord, and nerves.
Neurons (Nerve Cells)
Structure: A neuron has a cell body that contains the nucleus and cytoplasm, with long, thin, hair-like parts extending from it.
Axon: This is a single long extension responsible for sending signals.
Dendrites: These are many short, branched extensions that receive signals.
Length: A single neuron can be as long as a meter.
A neuron consists of a cell body, dendrites, and axons, which allow it to transmit nerve impulses.
Cells are the basic building blocks of all living organisms. They are the smallest units of life and can carry out all the necessary functions to sustain an organism. Just like how a house is made up of many rooms, a living organism is made up of many cells working together.
Discovery of Cells by Robert Hooke
In 1665, a scientist named Robert Hooke was looking at a thin slice of cork through a microscope he had designed himself.
He noticed that the cork looked like it was made up of many tiny compartments, similar to a honeycomb. Cork comes from the bark of a tree.
Hooke decided to call these tiny compartments “cells” (Latin word “cellula,” meaning a small room), and he observed dead cells in cork.
This observation was very important because it was the first time anyone had seen that living things seem to be made up of separate units.
What are Living Organisms Made Up of?
To find out, let’s Perform One Activity
Introduction:
The epidermis of an onion bulb is a single layer of cells that can be easily peeled off.
This thin layer of cells can be used to prepare a temporary mount for microscopic observation.
1. Peeling the Onion Skin: Using forceps, carefully peel off the epidermis from the concave side of the onion bulb.
2. Placing the Peel in Water: Immediately place the peeled layer in a watch glass containing water. This prevents the peel from folding or drying out.
3. Transferring the Peel to the Slide and then Preparing the Slide (i) Take a glass slide and put a drop of water on it. (ii) Transfer a small piece of the peel from the watch glass to the slide, ensuring it is flat. A thin camel hair paintbrush can help with this.
4. Adding Safranin and Staining the Peel (i) Add a drop of safranin solution to the peel. (ii) Carefully place a cover slip over the peel, using a mounting needle to avoid air bubbles.
5. Observing the Slide (i) You have now prepared a temporary mount of onion peel. (ii) Observe the slide under low power and then high power of a compound microscope to see the details of the onion cells.
6. Observation
(i) These structures look similar to each other. Together they form a big structure like an onion bulb! (ii) We find from this activity that onion bulbs of different sizes have similar small structures visible under a microscope. (iii) The cells of the onion peel will all look the same, regardless of the size of the onion they came from. (iv) These small structures that we see are the basic building units of the onion bulb. (v) These structures are called cells. (vi) Not only onions, but all organisms that we observe around us are made up of cells. (vii) However, there are also single cells that live on their own.
Scientists’ Contributions in Cell Discovery and Theory
Robert Hooke (1665). Discovered cells by observing a cork slice under a primitive microscope.
Antonie van Leeuwenhoek (1674). Using an improved microscope, he was the first to observe free-living cells in pond water.
Robert Brown (1831). Identified the nucleus within the cell.
Jan Evangelista Purkinje (1839). Coined the term ‘protoplasm’ for the cell’s fluid substance.
Matthias Schleiden (1838) and Theodor Schwann (1839). Proposed the cell theory, stating that all plants and animals are made of cells and that the cell is the basic unit of life.
Rudolf Virchow (1855). Expanded the cell theory by asserting that all cells arise from pre-existing cells.
Electron Microscope (1940). Enabled the observation and understanding of the complex structure of cells and their various organelles.
Try yourself:Who discovered the cell?
A.Robert Hooke
B.Purkinje
C.Robert Brown
D.None of these
Types of Organisms
Based on the number of cells, organisms are classified into two categories:
(a) Unicellular Organisms: These are single-celled organisms that carry out all life functions independently. Examples include Amoeba, Paramecium, Chlamydomonas, and various types of bacteria.
(b) Multicellular Organisms: These organisms are made up of many cells that work together, each taking on different roles to form various body parts. Examples include fungi, plants, and animals.
The shape and size of cells relate to their specific functions. There is a division of tasks among cells. Interestingly, most eukaryotic cells contain similar organelles, no matter their function or the type of organism they belong to.
Interesting facts and Functions of Cells in Human Body
Nerve Cell: Longest cell in the human body; carries messages across the body.
Blood Cells: RBCs (red blood cells) carry oxygen and lack nucleus (in mammals). – WBCs (white blood cells) fight infection.
Smooth Muscle Cell: Involuntary in action; helps organs like intestines to contract and relax.
Bone Cell: Stores calcium; helps build and maintain strong bones.
Fat Cell: Stores energy as fat; provides insulation and cushioning.
Ovum (Egg Cell): Largest human cell; carries mother’s DNA and supports early development.
Sperm: Smallest human cell with a tail; fertilizes the ovum to begin reproduction.
Various cells from the human body
We already know that all living organisms are made up of cells. But have you ever wondered what’s inside a cell that allows it to do so many important jobs? Let’s find out and explore the tiny parts inside a cell that help it stay alive and work properly.
What is a Cell Made Up of? What is the Structural Organisation of a Cell?
Almost every cell has three basic parts: plasma membrane, nucleus, and cytoplasm. All cell activities and interactions depend on these.
(a) Plasma Membrane or Cell Membrane
The plasma membrane is the outer layer of the cell that separates its contents from the outside environment. It allows certain materials to enter and exit the cell while blocking others, making it a selectively permeable membrane.
How does the movement of substances take place into the cell?
Types of Movement
Diffusion (For movement of gas): This is the natural movement of a substance from an area of high concentration to an area of low concentration. Gases like carbon dioxide or oxygen can pass through the cell membrane by diffusion. Other molecules require energy for transport in and out of the cell.
Osmosis (For Movement of Water): Osmosis is the diffusion of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration.
Do you know: Osmosis is special case of diffusion
The movement of water across the plasma membrane is also affected by the amount of substance dissolved in water. Thus, osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration.
What happens if we place an animal or plant cell in a sugar or salt solution? One of three outcomes may occur:
Hypotonic Solution: If the surrounding solution has a higher concentration of water than the cell, the cell will absorb water through osmosis. This is known as a hypotonic solution, leading to more water entering the cell than leaving it.
Isotonic Solution: If the surrounding solution has the same concentration of water as the cell, there will be no net movement of water across the cell membrane. This is an isotonic solution, where water moves in and out at equal rates.
Hypertonic Solution: If the surrounding solution has a lower concentration of water than the cell, the cell will lose water through osmosis. This is a hypertonic solution, causing the cell to shrink as water exits.
Examples and facts of Diffusion and Osmosis
1. A de-shelled egg placed in pure water swells because water enters the egg by osmosis. The same egg placed in concentrated salt solution shrinks as water moves out of the egg.
2. Dried raisins or apricots swell in plain water as they gain water by osmosis. In concentrated sugar/salt solution, they shrink due to loss of water.
3. Osmosis is important in plant roots and unicellular freshwater organisms for absorbing water.
4. Diffusion is important for exchange of gases and water in and out of the cell.
5. The plasma membrane, made of lipids and proteins, controls the movement of substances in and out of the cell, and its flexibility allows the cell to engulf food by endocytosis, as seen in Amoeba.
(b) Cell Wall
In addition to the plasma membrane, plant cells have a tough outer layer called the cell wall. This wall is located outside the plasma membrane and is mainly made of cellulose, a complex substance that gives plants their structural strength.
When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.
Function of Cell Wall
Cell walls allow plant, fungal, and bacterial cells to endure very dilute (hypotonic) external solutions without bursting. In such solutions, cells absorb water through osmosis, causing them to swell and exert pressure against the cell wall. The wall then pushes back, creating equal pressure.
Due to their walls, plant cells can handle much larger changes in their environment compared to animal cells.
Plasmolysis: This occurs when a living plant cell loses water by osmosis, resulting in the cell contents shrinking away from the cell wall.
Try yourself:
What is the function of the plasma membrane in a cell?
A.Allows the entry and exit of some materials
B.Prevents the movement of all materials
C.Only allows the entry of water
D.Only allows the exit of water
(c) Nucleus
The nucleus has a double layer called the nuclear membrane.
The nuclear membrane has pores that allow materials to move between the nucleus and the cytoplasm.
The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
Chromosomes hold the information needed to pass traits from parents to their children in the form of DNA (Deoxyribonucleic Acid).
Chromosomes are made up of DNA and protein.
DNA molecules carry the information required for building and organising cells.
Functional parts of DNA are known as genes.
In a non-dividing cell, DNA exists as chromatin material, which looks like a tangled mass of thread-like structures.
When the cell is ready to divide, the chromatin organises into chromosomes.
The nucleus is vital for cell reproduction, which is when a single cell divides to form two new cells.
The nucleus also plays an important role, along with the environment, in guiding how the cell develops and what it looks like when it matures.
It directs the chemical activities of the cell.
Nucleus of a Eukaryotic cell
Prokaryotic and Eukaryotic Cells
Prokaryotic Cells
Prokaryotes are organisms with cells that do not have a nuclear membrane.
Typically, prokaryotic cells are small, ranging from 1 to 10 µm in size.
They contain a nucleoid, which is an area with nucleic acids but lacks a surrounding membrane.
Prokaryotic cells do not have membrane-bound organelles.
They possess a single chromosome made of nucleic acid.
Also read: Short and Long Answer Questions- The Fundamental Unit Of Life
Eukaryotic Cells
Eukaryotes are organisms with cells that have a nuclear membrane.
Eukaryotic cells are generally larger, measuring from 5 to 100 µm.
They have a defined nuclear membrane and contain multiple chromosomes.
Eukaryotic cells include complex organelles that perform specific functions.
Q: Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells.
Cytoplasm
The cytoplasm is the fluid inside the plasma membrane.
It contains various specialised cell organelles, each with specific roles.
The cytoplasm facilitates the exchange of materials between organelles.
The cytoplasm is where certain metabolic pathways, such as glycolysis, take place.
Try yourself:What do chromosomes carry information for?
A.Creating energy
B.Storing nutrients
C.Cell division
D.Passing traits
Cell Organelles
Organelles are specialised structures that perform different tasks within cells. The term literally means “little organs.” Just as organs like the heart, liver, stomach, and kidneys have specific functions to keep an organism alive, organelles have specific roles to support the life of a cell. Membrane-bound organelles are a feature of eukaryotic cells and absent in prokaryotes.
1. Endoplasmic Reticulum (ER)
The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets that resemble long tubules or round bags called vesicles. The structure of the ER membrane is similar to that of the plasma membrane, made up of lipids and proteins. There are two types of ER: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).
Although the ER can look different in various cells, it consistently forms a network system.
Endoplasmic Reticulum
Types of Endoplasmic Reticulum
Functions of Rough and Smooth Endoplasmic Reticulum
The RER contains ribosomes, which are the sites for protein production. These proteins are transported to different parts of the cell as needed.
The SER aids in creating lipids, which are essential for cell function.
Some proteins and lipids serve as enzymes and hormones.
The ER acts as channels, transporting materials (especially proteins) within the cytoplasm or between the cytoplasm and the nucleus.
The endoplasmic reticulum provides a cellular framework in the cytoplasm, supporting specific cell activities.
In vertebrate liver cells, the SER is vital for detoxifying various poisons and drugs.
2. Golgi Apparatus
The Golgi apparatus is made up of a system of membrane-bound vesicles that are arranged in stacks known as cisterns. These membranes often connect with the membranes of the ER, forming part of a complex cellular membrane system. Golgi Apparatus
Function of the Golgi Body
The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus.
Its functions include the storage, modification and packaging of products in vesicles. In some cases, complex sugars may be made from simple sugars in the Golgi apparatus.
The Golgi apparatus is also involved in the formation of lysosomes.
3. Lysosomes
Lysosomes act as the waste disposal system of the cell. They have a membrane-bound structure and contain digestive enzymes made by the rough endoplasmic reticulum (RER).
Lysosome
Functions of Lysosomes
Lysosomes break down foreign materials entering the cell, like bacteria or food, and also old organelles into smaller pieces.
They contain powerful digestive enzymes that turn complex substances into simpler ones.
They break down old organelles.
If the cell is damaged, lysosomes may burst and the enzymes can digest their own cell. Hence, lysosomes are sometimes called the ‘suicide bags‘ of a cell.
4. Mitochondria
Mitochondria are known as the powerhouse of the cell.
Mitochondria
Structure of Mitochondria
Mitochondria have two membrane coverings.
The outer membrane is very porous, while the inner membrane has deep folds. These folds create a large surface area for ATP-generating chemical reactions.
Functions of Mitochondria
Mitochondria release energy needed for various chemical processes required for life in the form of ATP (Adenosine triphosphate) molecules. ATP is recognised as the energy currency of the cell.
The body uses energy stored in ATP to create new chemical compounds and for mechanical work.
Mitochondria are unique as they have their own DNA and ribosomes, allowing them to produce some of their own proteins.
Try yourself:
What is the function of cell organelles?
A.Store energy
B.Help cells function
C.Create food
D.Protect from diseases
Also read: Short and Long Answer Questions- The Fundamental Unit Of Life
5. Plastids
Plastids are found only in plant cells.
There are two types of plastids:
Chromoplasts (coloured plastids).
Leucoplasts (colourless plastids) mainly store materials like starch, oils, and protein granules.
Plastid
Structure of Plastids
The plastids’ internal structure includes multiple layers of membranes surrounded by a substance known as the stroma.
Plastids also have their own DNA and ribosomes, like mitochondria, and are similar to mitochondria in their structure
Function of Plastids
Chromoplasts containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants.
Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.
6. Vacuoles
Vacuoles are storage sacs that hold solid or liquid materials. In animal cells, they are typically small, while in plant cells, they can be quite large. The central vacuole in some plant cells may take up 50-90% of the cell’s volume.
In plant cells, vacuoles are filled with cell sap, which helps the cell maintain its shape and firmness. Vacuoles store many important substances for the plant cell, such as: Amino acids, Sugars, Various organic acids andSome proteins.
In single-celled organisms like Amoeba, the food vacuole holds the food that the Amoeba has eaten. Some unicellular organisms have special vacuoles that are important for: (i) removing excess water from the cell and (ii) getting rid of certain wastes.
Cell Division
The process of creating new cells is known as cell division. New cells are produced in living organisms to:
grow
replace old, dead, and damaged cells
form gametes needed for reproduction
There are two main types of cell division: mitosis and meiosis.
Mitosis or Mitotic Cell Division
Mitosis is the process through which most cells divide for growth. In this process, each original cell, known as the mother cell, splits to create two genetically identical daughter cells (Fig. 5.7). The daughter cells have the same number of chromosomes as the mother cell. This process is essential for the growth and repair of tissues in organisms.
MITOSIS
Meiosis or Meiotic Cell Division
In animals and plants, certain cells in reproductive organs or tissues divide to create gametes, which will develop into offspring after fertilisation. This division occurs through a different method called meiosis, which involves two successive divisions. When a cell undergoes meiosis, it produces four new cells instead of just two (Fig. 5.8). These new cells contain only half the number of chromosomes compared to the mother cells.
MEIOSIS
Difference between an Animal Cell and a Plant Cell
The structure of an atom consists of protons, neutrons, and electrons. Protons and neutrons each have a mass of one unit, while the mass of an electron is so small that it is often ignored. These fundamental components determine the mass and charge of the atom.
Atomic structure is about how these subatomic particles—protons, neutrons, and electrons—are arranged within an atom, which affects its composition and behaviour.
Structure of Atom
John Dalton believed that the atom cannot be divided.
In 1886, E. Goldstein found new radiations in a gas discharge tube, naming them canal rays. These rays carry a positive charge.
In 1897, J.J. Thomson discovered the electron, a subatomic particle with a negative charge.
The neutron was discovered by Chadwick and has no charge.
Let’s Revise: Why is the mass of the electron usually ignored?
Ans: Because it is approximately 1/1836 the mass of a proton, making it negligible.
Thomson’s Model of an Atom
Thomson’s Model of the Atom, referred to as the plum pudding model, suggested that the atom is made up of a positively charged sphere with negatively charged electrons scattered throughout it, akin to currants in a Christmas pudding. Another way to picture it is like a watermelon, where the positive charge is spread out like the red fruit, and the electrons are like seeds embedded within.
Electrons are embedded in a positively charged sphere; overall atom is neutral.
The negative and positive charges are balanced, leading to an atom that is overall electrically neutral.
Plum Pudding Model
Try yourself:
What does Thomson’s Model of the Atom compare to a watermelon?
A.The color
B.The rind
C.The juice
D.The seeds
Rutherford’s Model of an Atom
Rutherford’s Model of the Atom brought forth the concept of a small, dense nucleus at the centre of the atom, with electrons moving around it, which greatly changed our understanding of atomic structure.
Rutherford’s Experiment
α-particles are He2+ nuclei (mass ≈ 4 u, charge +2e) emitted at high speeds, hence have high kinetic energy.
Most of the atom’s interior is empty, as many α-particles went through the gold foil without deflecting.
A few particles were deflected, suggesting that the positive charge of the atom takes up very little space.
A tiny number of α-particles were deflected back by 180°, showing that the positive charge and mass of the gold atom are concentrated in a very small area.
Conclusions made by Rutherford
He calculated that the nucleus’s radius is about 100,000 times smaller than that of the atom.
The Nuclear Model of an Atom proposed by Rutherford includes:
A positively charged centre called the nucleus, where nearly all the mass of the atom is found.
The electrons orbit the nucleus in circular paths.
The nucleus is very small compared to the atom’s overall size.
Rutherford’s Nuclear Model of Atom
Drawbacks of Rutherford’s Model of the Atom
The orbiting electron in a circular path should not be stable. Any particle in such an orbit would experience acceleration. During this acceleration, charged particles would lose energy by radiating it. Therefore, the electron would lose energy and eventually spiral into the nucleus. If this were true, atoms would be highly unstable, which contradicts the fact that matter exists in a stable form.
Try yourself:Rutherford’s ‘alpha (α) particles scattering experiment’ resulted in the discovery of
A.electron
B.proton
C.nucleus in the atom
D.atomic mass
Bohr’s Model of Atom
Bohr’s Model of the Atom changed how we understand atomic structure by introducing the idea that electrons move around the nucleus in specific energy levels. This model helps explain why atoms are stable and how they produce spectral lines.
Historical Context of Niels Bohr
Niels Bohr (1885-1962) was born in Copenhagen on 7 October 1885. He became a professor of physics at Copenhagen University in 1916 and won the Nobel Prize for his contributions to atomic structure in 1922. Some of his important writings include:
The Theory of Spectra and Atomic Constitution
Atomic Theory
The Description of Nature
Also read: NCERT Solutions: Structure of the Atom
Postulates of Niels Bohr
Only certain specific orbits, called discrete orbits, are allowed for electrons inside the atom.
Electrons do not emit energy while they are moving in these discrete orbits.
These orbits or shells are referred to as energy levels. Energy levels in an atom are illustrated in Fig. 4.3.
Drawbacks of Bohr’s Model of Atom
Works for hydrogen but fails for multi-electron atoms.
Cannot explain the splitting of spectral lines (fine structure; effects in magnetic/electric fields).
Does not account for line intensities in spectra.
Assumes fixed circular orbits; later quantum model uses orbitals (no fixed paths).
Neutrons
In 1932, J. Chadwick discovered a subatomic particle with no charge, which has a mass almost equal to that of a proton. This particle is called a neutron. Neutrons are found in the nucleus of all atoms, excepthydrogen-1 (protium). Deuterium and tritium contain neutrons. Generally, a neutron is denoted as ‘n’. The mass of an atom is the total of the masses of the protons and neutrons in the nucleus.
Bohr’s Model
Let’s Revise: How is a hydrogen atom different from atoms of all other elements?
Ans: All atoms consist of three subatomic particles: electrons, protons, and neutrons. The hydrogen atom contains only one electron and one proton, and it has no neutrons, making it unique among all elements.
Distribution of Electrons in Different Orbits
The way electrons are arranged in various orbits, or energy levels, defines an atom’s electron configuration.
Rules
The maximum number of electrons that can fit in a shell is determined by the formula 2n², where ‘n’ represents the orbit number or energy level index (1, 2, 3, …).
The outermost orbit can hold a maximum of 8 electrons.
Fill shells step-wise (K → L → M …); outermost shell holds at most 8 electrons even if 2n2 allows more.
Thus, the maximum number of electrons in various shells is as follows:
First orbit or K-shell can hold = 2 electrons
Second orbit or L-shell can hold = 8 electrons
Third orbit or M-shell can hold = 18 electrons
Fourth orbit or N-shell can hold = 32 electrons
The atomic structure of the first eighteen elements is illustrated in a diagram.
The electrons in the outermost shell of an atom are called valence electrons. The number of valence electrons is essential in defining the chemical properties of the element.
Valency
Atomic structure of the first eighteen elements
An atom of each element has a definite combining capacity, called its valency.
The number of bonds that an atom can form in a compound is shown by its valency.
Valence electrons are the electrons in the outermost orbit of the atom.
Let’s Revise
Q: How is the maximum number of electrons in a shell calculated?
Ans: By the formula 2n², where n is the orbit number.
Q: Why are valence electrons important?
Ans: They determine the chemical properties and bonding behaviour of the element.
Also read: NCERT Solutions: Structure of the Atom
Atomic Number & Mass Number
The atomic number indicates the number of protons in an atom’s nucleus, represented by ‘Z’.
The mass number is the total count of protons and neutrons, giving information about the atom’s identity and mass.
The total number of protons in an atom’s nucleus is its atomic number, symbolised as ‘Z’.
The mass number of an atom is the sum of its protons and neutrons, represented by the letter ‘A’.
An element is represented as AXZ, where Z is the atomic number (equal to the number of protons), A is the mass number, and X is the element’s symbol. The mass number (A) can be calculated as: Mass number (A) = Number of protons (Z) + Number of neutrons.
Let’s Revise: What is the mass number?
Ans: The mass number of an element is the total of the number of protons and neutrons in the atom of that element.
Mass Number A = Number of protons + Number of neutrons.
For hydrogen, Z = 1, as there is only one proton in a hydrogen atom’s nucleus. Therefore, the mass number of H is 1.
Mass Number A refers to the total count of nucleons (protons and neutrons) in the nucleus.
Isotopes
Atoms of the same element with the same atomic number but different mass numbers are called isotopes. For example, hydrogen has three isotopes: protium (H), deuterium (²H or D), and tritium (³H or T).
Chemical properties → same
Physical properties → differentIsotopes of HydrogenApplications of Isotopes: (a) An isotope of Uranium is used as fuel in nuclear reactors. (b) An isotope of Cobalt is used in the treatment of cancer. (c) An isotope of Iodine is used in the treatment of goitre.
Try yourself:The number of electrons in a neutral atom of an element X is 15, and the number of neutrons is 16. Which of the following is the correct representation of the element?
A. 31X15
B. 31X16
C.16X15
D. 15X16
Isobars
Atoms of different elements that have the same mass number but different atomic numbers are called isobars. For example, Argon-40 (₁₈Ar⁴⁰) and calcium-40 (₂₀Ca⁴⁰) are isobars. Examples of Isobars
Let’s Revise
Q: What are General Features of Isotopes?
Ans:The general features of isotopes are:
Isotopes of an element have the same atomic number, meaning they have the same number of protons and electrons.
They have different mass numbers, resulting from a different number of neutrons.
The chemical properties of isotopes are similar, but their physical properties differ.
Different masses lead to variations in physical properties like melting point, boiling point, and density.
Q: What are Isotones?
Ans. Some atoms of different elements have different atomic numbers and different mass numbers but they have a same number of neutrons. These atoms are known as isotones.
Example:14C6 and 16O8.
Both C and O have the same number of neutrons i.e. 8.
Vermicomposting
Fertilisers
Irrigation
Cattle Farming
Inland Fisheries
Compression (C) & Rarefaction (R) of sound 
Man producing echo
Reverberation of Sound
Examples of Centripetal Force
Gravitation Formula
Free Fall Motion
Mass and Weight
(at a given place). For this reason, at a given place, we may use the weight of an object as a measure of its mass.
Thrust
Buoyancy
Push or Pull is called Force
Effects of Force
Balanced and Unbalanced Forces
Balanced Force
Illustration of Force depending on Mass and Acceleration
Motion
Types of Motion
Motion along a straight line
Example of Zero Displacement
Given:
(ii) s/t graph for non-uniform motion:
v = (s2 – s1)/(t2 – t1)
a = (v2 – v1)/(t2 – t1)
a2 ≠ a1
a1‘ = a2‘
Tissue
Intercalary meristem section 
Parenchyma Tissue in Transverse and Longitudinal section
Sclerenchyma Tissue
Epidermis
Xylem Elements
Working of Xylem and Phloem in plant
Section of phloem
Various cells from the human body
Golgi Apparatus
Lysosome
Mitochondria
Plastid
MITOSIS
MEIOSIS
Structure of Atom
Plum Pudding Model
Rutherford’s Experiment
Bohr’s Model
Atomic structure of the first eighteen elements
Examples of Isobars