Chapter Notes: Sound

Understanding Sound

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:
  1. Strike the tuning fork against the rubber pad to make it vibrate.
  2. Hold the vibrating fork near your ear and listen to the sound.
  3. Touch a vibrating prong with your finger and feel the vibrations.
  4. Hang a small ball using a thread. Lightly touch the ball with the vibrating fork and watch how it moves.
• Observations:
  1. The vibrating fork makes sound.
  2. Touching the prong lets you feel the vibrations.
  3. 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.

MULTIPLE CHOICE QUESTION

Try yourself: What is vibration?View Solution

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.

MULTIPLE CHOICE QUESTION

Try yourself: In longitudinal waves, how do particles of the medium move in relation to the direction of wave propagation?View Solution

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.

MULTIPLE CHOICE QUESTION

Try yourself: What is the definition of sound?View Solution

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

MULTIPLE CHOICE QUESTION

Try yourself: What is the minimum time interval required for a distinct echo to be heard?View Solution

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.

MULTIPLE CHOICE QUESTION

Try yourself: What is the upper limit of the audible range for children under five years old and some animals?View Solution

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.

Chapter Notes: Work and Energy

Work, Power, and Energy

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 EnergyKinetic 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:

• E_k 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 (E_p) of the object.

E_p = 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 (E_p) of the object.

Potential Energy (E_p) = 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: E_p = 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.

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.

Chapter Notes: Force and Laws of Motion

Introduction to Motion and ForceMotion 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.

Illustration of 3rd Law of Motion.

Applications

• Walking is enabled by the 3rd law.
• A boat moves back when we deboard it.
• A gun recoils.
• Rowing of a boat.

Chapter Notes: Motion

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

MULTIPLE CHOICE QUESTION

Try yourself: What is the definition of distance?View Solution

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).

MULTIPLE CHOICE QUESTION

Try yourself: What is the difference between distance and displacement?View Solution

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 (v_avg) = (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

MULTIPLE CHOICE QUESTION

Try yourself: What is the definition of uniform motion?View Solution

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

MULTIPLE CHOICE QUESTION

Try yourself: Which quantity requires both magnitude and direction.View Solution

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

Watch the animated video below to understand the concepts in an easy manner.

https://youtube.com/watch?v=ZM8ECpBuQYE%3Fwmode%3Dopaque

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. v_A > v_B

MULTIPLE CHOICE QUESTION

Try yourself: Which of the following statements is true about acceleration?View Solution

Equations of Motion by Graphical Method

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

MULTIPLE CHOICE QUESTION

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?View Solution

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

Chapter Notes: Atoms and Molecules

Introduction

Atoms and molecules are the fundamental building blocks of matter. A clear understanding of atoms and molecules explains why substances combine in particular ways, why physical and chemical properties differ from one substance to another, and how new substances are formed in chemical reactions.

• Maharishi Kanad and Pakudha Katyayama in ancient India proposed that matter can be divided into smaller indivisible particles called Parmanu.
• Democritus and Leucippus in ancient Greece proposed a similar idea: matter is made of indivisible particles called atoms.
• These early ideas were philosophical and lacked experimental proof until modern chemistry developed in the 18th century.
• In the late 18th century, Antoine L. Lavoisier established quantitative methods in chemistry and laid foundations for modern chemical science by formulating laws about chemical combinations.
• Lavoisier and Joseph L. Proust performed careful experiments that led to two important laws: the Law of Conservation of Mass and the Law of Constant Proportions (also called the Law of Definite Proportions).
• These laws guided later work and helped John Dalton formulate his atomic theory that explained why these laws hold true for chemical reactions.

Try yourself:

Who proposed the idea that matter can be divided into smaller particles called Parmanu?

• A.Maharishi Kanad
• B.Democritus
• C.Antoine L. Lavoisier
• D.Joseph L. Proust

Laws of Chemical Combination

Two fundamental laws describe how substances combine in chemical reactions: the Law of Conservation of Mass and the Law of Constant Proportions. These laws provide the experimental basis for the atomic view of matter.

1. Law of Conservation of Mass

The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.

Example of Law of conservation of Mass

Careful experiments – for example, mixing chemical solutions in closed containers and measuring mass before and after reaction – show that the measured total mass remains unchanged, supporting this law.

Try yourself:

According to the Law of Conservation of Mass, what happens to the mass during a chemical reaction?

• A.The mass increases.
• B.The mass decreases.
• C.The mass remains constant.
• D.The mass is converted into energy.

2. Law of Constant Proportion

The Law of Constant Proportion (Law of Definite Proportions) states that a chemical compound always contains the same elements in the same fixed proportion by mass, irrespective of its source.

Example of Law of Constant Proportion

For example, pure water always contains hydrogen and oxygen in the mass ratio 1 : 8. This proportion is the same whether the water comes from a river, a well, or rain.John Dalton’s Atomic Theory

To explain these laws, John Dalton proposed an atomic theory which gave an experimental and conceptual basis for atoms and compounds.

John Dalton

Postulates of Dalton’s Atomic Theory

1. All matter is made of extremely small particles called atoms.
2. Atoms are indivisible by chemical means and remain unchanged in chemical reactions.
3. Atoms of the same element are identical in mass and properties.
4. Atoms of different elements have different masses and properties.
5. Atoms combine in simple whole-number ratios to form compounds.
6. The relative number and types of atoms in a given compound are constant (fixed composition).

Background on John Dalton

• John Dalton was born in 1766. His atomic hypothesis explained the Law of Conservation of Mass and the Law of Definite Proportions quantitatively and conceptually.

What is an Atom?

An atom is the smallest particle of an element that retains the chemical properties of that element and cannot be broken down by chemical means.

• Atoms are extremely small; a very large number of atoms are required to form visible matter.
• A layer only a few million atoms thick may be comparable in thickness to a sheet of paper.

Atomic Radius

The atomic radius is a measure of the size of an atom, typically expressed in nanometres (nm), where 1 nm = 10^-9 m.

Try yourself:

Which statement best describes the Law of Constant Proportion?

• A.It states that in a chemical substance, elements are always present in definite proportions by volume.
• B.It states that in a chemical substance, elements are always present in definite proportions by mass.
• C.It states that in a chemical substance, elements are always present in indefinite proportions by mass.
• D.It states that in a chemical substance, elements can be present in any proportions by mass.

Modern Symbols of Elements

Element symbols evolved from early pictorial symbols to the simple one- or two-letter symbols used today. The International Union of Pure and Applied Chemistry (IUPAC) standardises these symbols.

• Historical background: John Dalton first used symbols to represent atoms. Later, Berzelius proposed using one or two letters derived from the element name to represent elements.
• Origin of element names: Some names come from places (e.g., copper from Cyprus) or from colours and other properties.
• Modern symbols: Most element symbols are derived from their English names; the first letter is capitalised and a second letter, if used, is lowercase.
• First letter + another letter: Examples include Chlorine: Cl, Zinc: Zn.
• Names from other languages: Some symbols come from Latin, Greek or other languages: Iron: Fe (ferrum), Sodium: Na (natrium), Potassium: K (kalium).

Symbols for Some Elements

Try yourself:

Which scientist pioneered the use of symbols for elements?

• A.Berzelius
• B.Dalton
• C.IUPAC
• D.None of the above

Atomic Mass

Atomic mass of an atom is the mass of that atom expressed relative to a standard. The standard used internationally is defined as 1/12 of the mass of a carbon-12 atom. The unit for atomic mass is the unified atomic mass unit (symbol u, also called amu).

• Atomic mass is the combined mass of protons, neutrons and electrons in an atom; in practice the electron mass is very small relative to protons and neutrons and often neglected in simple calculations.
• Atomic masses reported on the periodic table are average values that reflect the natural isotopic composition of the element.

Atomic mass of some elementsHow Do Atoms Exist?

• Many atoms do not exist freely under normal conditions; they combine to form molecules or form ions that aggregate into ionic structures.
• Visible matter is composed of huge numbers of molecules or ionic units assembled together.

What is a Molecule?

A molecule is the smallest particle of an element or compound that can exist independently and retain the chemical properties of that substance.

Molecules of Elements

1. Monoatomic molecules: Some elements exist as single atoms (monoatomic) in their natural gaseous state, e.g., Helium (He), Argon (Ar).
2. Diatomic molecules: Several non-metals exist as molecules of two atoms, e.g., Hydrogen (H₂), Oxygen (O₂), Nitrogen (N₂), Chlorine (Cl₂).
3. Polyatomic molecules: Some elements form molecules with more than two atoms, e.g., Phosphorus (P₄), Sulphur (S₈).

Try yourself:What is the atomic mass of an atom?

• A. The total mass of the neutrons and protons in an atom.
• B.The mass of a carbon-12 atom in its ground state.
• C.The average mass of a group of atoms.
• D.The mass of an atomic particle.

Atomicity

Atomicity is the number of atoms present in one molecule of an element.

Atomicity of some non-metalsMolecules of Compounds

When atoms of different elements combine, they form molecules of compounds. These molecules have fixed compositions and properties different from their constituent elements.

What is an Ion?

• An ion is an atom or a group of atoms that carries an electric charge due to loss or gain of electrons.
• A positively charged ion is called a cation; a negatively charged ion is called an anion.
• Compounds formed from metals and non-metals often contain ions; such compounds are called ionic compounds.
• Polyatomic ions are groups of atoms bonded together that carry a net charge, for example, NO₃^–, SO₄^2-, OH^–.

Writing Chemical Formulae

• The chemical formula of a compound shows which elements are present and the number of atoms of each element in the smallest unit of that compound.
• To write formulae, you must know element symbols and the valencies (combining capacities) or ionic charges of the atoms/ions involved.
• Valency indicates how many electrons an atom gains, loses or shares when it forms a compound.
• Think of valency as the number of bonds an atom typically forms: it is the atom’s “combining power”.

Rules for Formula Writing

• The total positive charge and total negative charge in a neutral compound must balance.
• When writing formulae for compounds of a metal and a non-metal, write the metal first and the non-metal second (e.g., CaO, NaCl).
• Use the simplest whole-number ratio of atoms or ions that balances charges.
• When polyatomic ions are present in more than one number, enclose the polyatomic ion in brackets and write the number outside the brackets, e.g., Mg(OH)₂.
• Practice with examples to become familiar with common valencies and formulas.

Try yourself:What is the atomicity of a molecule?

• A.The number of atoms in a molecule
• B.The number of ions in a molecule
• C.The number of elements in a molecule
• D.The number of protons in a molecule

Formulae of Simple Compounds

Binary compounds (formed by two elements) can be written by criss-crossing valencies or balancing charges of ions.

Example:

• Carbon tetrachloride, CCl₄: Carbon (valency 4) combines with chlorine (valency 1) to give the formula CCl₄.

• Magnesium chloride, MgCl₂: Magnesium (valency 2) combines with chlorine (valency 1) to give MgCl₂.

Molecular Mass

The molecular mass (relative molecular mass) of a molecule is the sum of the atomic masses of all atoms present in the molecule. It is expressed in atomic mass units (u).

Example 1:

(a) Calculate the relative molecular mass of water (H₂O).

(b) Calculate the molecular mass of HNO₃.

Solution:

(a)

Atomic mass of hydrogen = 1 u.

Atomic mass of oxygen = 16 u.

The molecular mass of H₂O = 2 × (atomic mass of H) + 1 × (atomic mass of O).

The molecular mass of H₂O = 2 × 1 + 16 = 18 u.

(b)

Atomic mass of hydrogen = 1 u.

Atomic mass of nitrogen = 14 u.

Atomic mass of oxygen = 16 u.

The molecular mass of HNO₃ = 1 × (atomic mass of H) + 1 × (atomic mass of N) + 3 × (atomic mass of O).

The molecular mass of HNO₃ = 1 + 14 + 3 × 16 = 63 u.

Try yourself:What is the chemical formula for magnesium chloride?

• A.MgC₂
• B.ZnCl₂
• C.MgCl₂
• D.Mg

Formula Unit Mass

Formula unit mass is the sum of the atomic masses of the atoms present in the formula unit of an ionic compound. It is calculated the same way as molecular mass but applied to ionic formula units.

Example 2: Calculate the formula unit mass of CaCl₂.

Solution:

The atomic mass of Ca = 40 u.

The atomic mass of Cl = 35.5 u.

The formula unit mass of CaCl₂ = atomic mass of Ca + 2 × atomic mass of Cl.

The formula unit mass of CaCl₂ = 40 + 2 × 35.5 = 40 + 71 = 111 u.

Chapter Notes: Structure of the Atom

Atomic Structure

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?

View Answer  

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

​

MULTIPLE CHOICE QUESTION

Try yourself: What does Thomson’s Model of the Atom compare to a watermelon?View Solution

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 He$\mathrm{<sup\; style="box-sizing:\; border-box;\; font-family:\; Lato,\; sans-serif\; !important;\; letter-spacing:\; inherit;\; position:\; relative;\; font-size:\; 12px\; !important;\; line-height:\; 2\; !important;\; vertical-align:\; baseline;\; top:\; -0.5em;\; text-align:\; inherit;\; color:\; rgb(0,\; 0,\; 0)\; !important;\; word-break:\; break-word;">2</sup>}<sup\; style="box-sizing:\; border-box;\; font-family:\; Lato,\; sans-serif\; !important;\; letter-spacing:\; inherit;\; position:\; relative;\; font-size:\; 12px\; !important;\; line-height:\; 2\; !important;\; vertical-align:\; baseline;\; top:\; -0.5em;\; text-align:\; inherit;\; color:\; rgb(0,\; 0,\; 0)\; !important;\; word-break:\; break-word;">+</sup>$ 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.

MULTIPLE CHOICE QUESTION

Try yourself: Rutherford’s ‘alpha (α) particles scattering experiment’ resulted in the discovery ofView Solution

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

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, except hydrogen-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?

View Answer  

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?

View Answer  

Q: Why are valence electrons important?

View Answer  

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 ^AX_Z, 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?

View Answer  

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.

MULTIPLE CHOICE QUESTION

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?View Solution

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

Short Notes: Is Matter Around Us Pure

Pure Substances

A pure substance is a material composed of only one type of particle (atom or molecule) and has a fixed composition and properties. It cannot be separated into other kinds of matter by any physical process. Elements and compounds are examples of pure substances. 

Mixtures Mixtures are made up of two or more elements or compounds mixed in any ratio/proportion.

Properties

• It may be homogeneous or heterogeneous.
• The properties of constituent substances are retained.
• No new compound is formed after mixing.
• Constituents of a mixture can be separated by simple physical processes.
• It does not have a fixed melting and boiling point.

Try yourself:Mixture can be       

• A.Homogeneous
• B.Heterogeneous       
• C.Both (a) and (b)
• D.Pure substance

Solution

A solution is a homogeneous mixture of two or more substances (e.g., Lemonade, soda water). It has two main components: the solute (the substance that gets dissolved) and the solvent (the substance that dissolves the solute).

Feature	Solute	Solvent
Definition	The substance that is dissolved in a solution	The substance that dissolves the solute
Quantity	Usually present in smaller amounts	Usually present in larger amounts
Example	Salt or sugar in water	Water (when salt or sugar is dissolved in it)
State	Can be solid, liquid, or gas	Can be liquid, but also sometimes a gas or a solid

Properties of a Solution 

• The particles of a solution are smaller than 1 nm (10^-9 metres) in diameter. So, they cannot be seen with the naked eye. 
• Because of their very small particle size, they do not scatter light passing through the solution. So, the path of light is not visible in a solution. 
• The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed; that is, a solution is stable.

Concentration of Solution

• The amount of solute that has dissolved in a specific amount of solvent or solution is measured as solution concentration.
• A concentrated solution is one that has a significant amount of dissolved solute in it. 
• A diluted solution is one that has a small amount of dissolved solute in it.

AlloysAlloys are homogeneous mixtures of metals or a mixture of a metal and another element that cannot be separated into its components by physical methods.

Examples:

• Steel – a combination of iron (metal) and carbon (non-metal).
• Bronze – a combination of copper (metal) and tin (metal).
• Brass – a mixture of copper (metal) and zinc (metal).

Suspension

A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Ex: Chalk in water, smoke in the air

Properties of Suspension :

1. It is a heterogeneous mixture.
2. Particles of a suspension are visible to the naked eye.
3. The size of the particles is greater than 1000 nm.
4. It is an unstable mixture. The solute settles down over a period of time.
5. If the solution is passed through filter paper, the solute and solvent get separated.
6. It scatters light when light is passed through the solution, i.e. it shows the Tyndall effect.

Types of Solution based on particle size

Colloidal Solution

A colloid solution is a heterogeneous mixture in which the size of particles lies between that of true solutions and suspensions. 

• Colloidal particles can easily scatter a beam of visible light. 
• This phenomenon is called the Tyndall effect. 

Tyndall Effect

Properties of Colloidal Solution:

1. The particles of colloids can’t be seen by the naked eye individually.
2. It is a heterogeneous mixture and thus the solute and solvent can’t be separated by filter paper.
3. The size of particles is smaller than suspensions but greater than solutions (1 nm to 100 nm).
4. It is a stable mixture. Particles do not settle down at the bottom over a period of time.

Try yourself:In sugar solution,       

• A.Sugar is solute, water is solvent
• B.Sugar is solvent, water is solute       
• C.Both are solutes
• D.Both are solvents.

Common Examples of Colloids

Physical and Chemical Change

Types of Pure Substances

The pure substance is divided into two types based on their chemical composition:

(i)  ElementsAccording to Antoine Laurent Lavoisier, an element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions. It is divided into three types, which are metals, non-metals and metalloids.

Properties of Metals

Examples: Gold, Silver, Copper, Iron, Sodium, Potassium etc. are Metals

Note: Mercury is the only metal that is liquid at room temperature.

Properties of Non-Metals

Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine etc.

Metalloids: Elements having intermediate properties between those of metals and non-metals are called metalloids.  Examples are boron, silicon, germanium etc.

Try yourself:

What is a solution?

• A.A mixture of two or more elements or compounds mixed in any ratio/proportion.
• B.A homogeneous mixture of two or more substances.
• C.A chemical reaction that forms a new compound.
• D.A mixture with a fixed melting and boiling point.

(ii) CompoundsA compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.

Characteristics

• The properties of a compound differ from those of its constituents.
• The compound has a fixed melting point and boiling point.
• A compound is a homogeneous substance.
• Constituent elements can be separated by chemical processes.

Difference between Compounds and Mixtures

Summary 

Chapter Notes: How do Organisms Reproduce

Introduction

“Have you ever wondered why organisms reproduce, even though it isn’t needed for day-to-day survival like breathing or eating? To understand why, imagine a world where no organisms reproduced—there would be no new plants, animals, or even humans. The world would be very different, right? That’s why reproduction is so important!”

When organisms reproduce, they create new individuals that look a lot like them. It’s like how you might resemble your parents.

• Reproduction is significant because it allows for the existence of large numbers of organisms belonging to a single species.
• If there was only one non-reproducing member of a particular kind, it is unlikely that we would have noticed its existence.
• Organisms belonging to the same species are often identified by their similarity in appearance.
• Reproducing organisms create new individuals that closely resemble themselves.

Do Organisms Create Exact Copies of Themselves?

• Organisms look similar because they share similar body designs, indicating a common source for these designs.
• Reproduction is the process through which these similar designs are created.
• The cell’s nucleus contains chromosomes, carrying information for inheriting features, present in the form of DNA molecules.
• DNA (Deoxyribonucleic acid)  in the nucleus is the source of information for protein synthesis, and changes in this information lead to altered body designs.
  DNA
• A fundamental event in reproduction involves the creation of a DNA copy.
• DNA copying results in the formation of an additional cellular apparatus, leading to the separation of DNA copies into two cells through cell division.
• As DNA copying is a biochemical process, variations may occur each time, making the process not entirely reliable.
• If the new DNA copy is not viable, the cell will not survive, and surviving cells may be similar but not identical, subtly differing from each other.

Try yourself:Why is reproduction significant for the existence and persistence of a species?

• A.It ensures the existence of large numbers of organisms belonging to a single species.
• B.It consumes energy but is necessary for survival.
• C.It allows for the creation of new individuals that closely resemble their parents.
• D.It leads to variations in body designs due to changes in DNA copying.

The Importance of Variation

• Reproduction’s consistent DNA copying is crucial to preserve an organism’s body design, allowing it to occupy a specific space or niche in the ecosystem.
• Reproduction is closely tied to maintaining the stability of a species population.
• Variations become significant as they help organisms adapt to new environmental conditions, ensuring survival.
• In situations with environmental changes, organisms with variations stand a better chance of adapting to new niches, maintaining the species over time.
• For example, if bacteria in temperate waters face a temperature rise due to global warming, variants resistant to heat may survive and prevent the extinction of the entire bacterial species.
• The survival of a species over time relies on the importance of variations.

Modes of Reproduction Used by Single Organisms

Reproduction can be defined as a process that involves the production of an offspring by a particular individual or individuals with the aim of propagating their species. Generally, reproduction happens during the reproductive phase of an organism. The mode of reproduction may vary in organisms. They can be broadly categorised as:

https://youtube.com/watch?v=n62_JysrwOw%3Fwmode%3Dopaque

1. Asexual Reproduction

Asexual reproduction is a mode of reproduction in which the new offspring arise from a single parent. The offsprings are identical to each other, both physically as well as genetically. They are the exact copies of their parent cell. Hence, they are ‘clones’. This mode of reproduction in both unicellular and multicellular organisms.

Types of Asexual Mode of Reproduction:

(a) Fission

Fission reproduction refers to a type of asexual reproduction in which an organism splits into two or more parts, and each part develops into a separate individual. The parent cell divides into daughter cells.

Single-celled organisms, like bacteria and amoebas, reproduce by dividing into new cells. Here’s how it works:

• Simple Division: Many bacteria and protozoa just split into two equal parts. It’s like cutting a cookie in half.
• Amoeba: Amoebas can divide in any direction, not just straight down the middle.

IBinary Fission in Amoeba 

• Organized Division: Some single-celled organisms, like Leishmania (which has a tail-like structure), divide in a specific direction related to this tail.

Binary Fission in Leishmania

• Multiple Division: Other organisms, like the malaria parasite Plasmodium, don’t just make two new cells; they split into many new cells all at once.

Multiple Fission In Plasmodium 

Let’s Revise: In which organisms does binary fission take place?

View Answer  

(b) Fragmentation

Fragmentation is a type of asexual reproduction where an organism breaks into fragments, and each fragment develops into a new individual. Example: Spirogyra.

• Simple Reproduction: In simple multi-cellular organisms like Spirogyra, reproduction can happen by breaking into smaller pieces. Each piece grows into a new organism.
• Complex Bodies: More complex organisms, like animals and plants, have specialized cells organized into tissues and organs.
• Specialized Cells: Because these complex organisms have many different cell types, they can’t just divide into smaller pieces to reproduce.
• Reproductive Cells: Instead, they use special cells designed for reproduction. These cells can grow and make all the different types of cells needed for the organism.
• Organism Growth: These specialized reproductive cells help create new organisms by producing and organizing all the different cell types required.

Let’s Revise: Why can’t complex organisms reproduce by fragmentation?

View Answer  

(c) Regeneration

• When an organism is cut or broken into multiple pieces, and each of those pieces has the ability to develop into a complete and functional organism, it is referred to as regeneration.
• Examples: Animals like Hydra and Planaria can be split into pieces, and each piece can become a new organism.
• Regeneration happens with the help of special cells. These cells quickly make many new cells. From this group of new cells, some change into different types of cells and tissues. This process of changing and organizing cells is called development.

Note: Not the Same as Reproduction: Regeneration is not the same as regular reproduction because most organisms don’t need to be cut to reproduce.

Q: How does regeneration in Hydra differ from typical reproduction?

View Answer  

(d) Budding

• In some organisms, a bud forms as a small outgrowth on the parent body. This bud undergoes development and gradually matures into a miniature individual. 
• Once matured, the bud detaches from the parent body and continues to grow and develop independently, eventually becoming a fully functional new individual. 
• This process is known as budding and is a form of asexual reproduction observed in various organisms.
  Example: Hydra

(e) Vegetative Propagation

This is the mode of reproduction by which plants reproduce asexually. In this mode, new plants are developed from a plant’s vegetative parts like stem, leaf, and root.

• Special Methods: Methods like layering and grafting help grow plants such as sugarcane, roses, and grapes.
• Faster Growth: Plants grown this way can produce flowers and fruits quicker than those grown from seeds.
• Seedless Plants: This method helps grow plants like bananas and roses that don’t produce seeds.
• Similar Traits: New plants are very similar to the parent plant and have the same characteristics.

Note: Leaf buds: This is a method in which the buds in the notches of leaves develop into new plants. This can be seen in bryophyllum.

Try yourself:Which of the following plants can be propagated through their leaves?

• A.Dahlia
• B.Sweet Potato
• C.Potato
• D.Bryophyllum

(f) Spore Formation

• Spores are small bulb-like structures that are covered by thick walls. Under favourable conditions, they germinate and produce new organisms.
• The thread-like structures on the bread are called hyphae and they belong to the bread mold Rhizopus. Hyphae are not reproductive parts.
• The tiny structures resembling blobs on a stick are called sporangia and they are involved in reproduction.
• Sporangia contain cells, called spores, which can develop into new individual Rhizopus organisms.
  Example: Rhizopus

2. Sexual Reproduction

Sexual reproduction refers to the process of reproduction in which the fusion of male and female gametes occurs, leading to the formation of genetically diverse offspring with unique combinations of traits.

Why the Sexual Mode of Reproduction?

• Sexual reproduction involves a male and a female to produce offspring, contributing to greater variations within a species due to the different patterns of variations in the individuals.
• This method combines DNA from two distinct individuals, creating unique combinations and variations, mixing the gene pool and promoting species survival through genetic recombination.
• Fertilisation restores the normal chromosome number because gametes contain half the number of chromosomes, but germ cells or gametes with half the chromosomes solve this issue.
• Germ cells, specialized for sexual reproduction, vary in complexity among organisms. In simpler organisms, germ cells are similar, while in more complex ones, they differentiate into female (storing food) and male (small and motile) gametes, contributing to the differences in male and female bodies and reproductive systems.

(a) Sexual Reproduction in Flowering Plants

• Sexual reproduction in plants involves the fusion of male and female gametes, found in the stamen and pistil of flowers.
• The stamen, or male reproductive part, consists of a filament and an anther enclosing pollen grains containing male gametes.
  
• The pistil, or female reproductive part, comprises the stigma, style, and ovary, with the ovary containing an ovule housing the egg cell.
• Flowers are classified as unisexual (incomplete flowers, having either stamens or pistils) or bisexual (containing both stamens and pistils).
• The process begins with pollination, where pollen grains transfer from the stamen’s anther to the pistil’s stigma, facilitated by agents like wind, birds, or animals.
• Pollination can be self-pollination (within the same flower) or cross-pollination (between flowers of the same species).
• Fertilization follows, involving the fusion of male and female germ-cells, forming a zygote that develops into an embryo inside the ovule.
• The ovule matures into a seed within the fruit as the flower’s other parts shed.
• Germination occurs when the seed, containing the embryo, develops into a seedling under suitable conditions, relying on stored nutrients in cotyledons and protected by a seed coat.
  

Try yourself:Which of the following is the correct order of events in the process of seed formation?

• A.Pollination, formation of embryo, fertilization, development of ovary into fruit
• B.Formation of pollen tubes, development of ovary into fruit, pollination, fertilization
• C.Pollination, fertilization, formation of embryo, development of ovary into fruit
• D.Development of ovary into fruit, pollination, fertilization, formation of embryo

(b) Reproduction in Human Beings

• Human reproduction is sexual, and the reproductive phase is when an individual is ready to have offspring, marked by changes starting from birth.
• Adolescence is a crucial phase for sexual maturation, leading to the development of specialized germ cells during puberty.
• Puberty brings noticeable changes like facial and body hair growth, voice deepening, sweat and sebaceous glands activation, and penis enlargement in boys.
• Girls experience pubic hair growth, breast enlargement, oily skin causing pimples, and the onset of menstruation during puberty.
• Both boys and girls become conscious of their changing appearances.
• Special organs like the penis in males and the uterus in females are essential for the actual transfer of germ cells in sexual reproduction.

Male Reproductive System

• The male reproductive system consists of organs that produce and transport the male germ-cell or gamete, male hormone testosterone and the organs which facilitate the discharge of male germ-cells into the female reproductive system for fertilization. 
• The male gamete is the sperm which is a tiny body containing the genetic material and they have a long tail for motility to help them reach the female germ-cell for fertilization. 
• The system consists of some external organs like penis, scrotum, testes and internal organs like urethra, prostate and seminal vesicles.

(i) Testes

• A pair of testes are located inside the scrotum which is present outside the abdominal cavity. 
• Scrotum has a relatively lower temperature needed for the production of sperm.
• Male germ cells i.e. sperms are formed here.
• Testes release the male sex hormone (testosterone).
• The function of testes:
  • Regulate the production of sperm.
  • Bring changes at puberty.

(ii) Vas deferens

• It passes sperm from the testes up to the urethra.

(iii) Urethra

• It is a common passage for both sperm and urine. Its outer covering is called the penis.

(iv) Associated glands 

• Seminal vesicles and prostate gland add their secretion to the sperms. This fluid provides nourishment to sperms and makes their transport easy.
• Sperm along with the secretion of glands form semen.

Let’s Revise: How does the scrotum contribute to sperm production in the male reproductive system?

View Answer  

Female Reproductive System

• The female reproductive system consists of organs responsible for producing female germ cells, facilitating gamete fertilization, and supporting embryo development into a new individual.
• Ovaries are the organs producing female gametes, known as eggs. The ovaries also generate hormones, including estrogen and progesterone, which trigger the development of secondary sexual characteristics in girls during puberty. 
• Components of this system include a pair of ovaries, oviducts, uterus, and the vagina opens externally. The urethra is a separate opening meant for urine.
  

(i) Ovary

• A pair of ovaries are located on both sides of the abdomen.
• Female germ cells i.e. eggs are produced here.
• At the time of birth of a girl, thousands of immature eggs are present in the ovary.
• At the onset of puberty, some of these eggs start maturing.
• One egg is produced every month by one of the ovaries.

(ii) Oviduct or Fallopian tube

• Receives the egg produced by the ovary and transfers it to the uterus.
• Fertilisation i.e. fusion of gametes takes place here.

(iii) Uterus

• It is a bag-like structure where the development of the baby takes place.
• The uterus opens into the vagina through the cervix.

Fertilisation of Egg

1. When the egg is fertilised

• The fertilized egg called a zygote is planted in the uterus and develops into an embryo.
• The embryo gets nutrition from the mother’s blood with the help of a special tissue called the placenta. It provides a large surface area for the exchange of glucose, oxygen and waste material.
• The period from fertilization up to the birth of the baby is called the gestation period. It is about 9 months.

2. When the egg is not fertilised

• The uterus prepares itself every month to receive fertilized eggs.
• The lining of the uterus becomes thick and spongy, required to support the embryo.
• When fertilisation had not taken place, this lining is not needed any longer.
• This lining breaks and comes out through the vagina as blood and mucus.
• This cycle takes around 28 days every month and is called menstruation.

Let’s Revise

Q: What is the role of the placenta in human reproduction?

View Answer  

Q: Why does menstruation occur in the absence of fertilisation?

View Answer  

Reproductive Health

Reproductive health means total well-being in all aspects of reproduction i.e. physical, emotional, social and behavioural.

Sexually Transmitted Diseases (STDs)

• Many diseases can be sexually transmitted such as:
  (i) Bacterial: Gonorrhoea and syphilis
  (ii) Viral: Warts and HIV-AIDS
• The use of condoms prevents these infections to some extent.

Contraception: It is the avoidance of pregnancy, which can be achieved by preventing the fertilisation of ovum.

Methods of contraception

(i) Physical barrier

• To prevent the union of egg and sperm.
• Use of condoms, cervical caps and diaphragm.

(ii) Chemical methods

• Use of oral pills
• These change the hormonal balance of the body so that eggs are not released.
• May have side effects.

(iii) Intrauterine contraceptive device (IUCD)

• Copper-T or loop is placed in the uterus to prevent pregnancy.

(iv) Surgical methods

• In males, the vas deferens are blocked to prevent sperm transfer called vasectomy.
• In females, the fallopian tube is blocked to prevent egg transfer called tubectomy.

Try yourself:Which of the following is a physical barrier method of contraception?

• A.Oral pills
• B.Intrauterine contraceptive device (IUCD)
• C.Vasectomy
• D.Condoms

Female Foeticide

• The practice of killing a female child inside the womb is called female foeticide.
• For a healthy society, a balanced sex ratio is needed which can be achieved by educating people to avoid malpractices like female foeticide and prenatal sex determination.
• Prenatal sex determination is a legal offence in our country to maintain a balanced sex ratio

Plant Tissues

Plant tissues are of two types on the basis of their dividing capacity:
(i) Meristematic tissue (growing tissue)
(ii) Permanent tissue

Meristematic Tissue

• These are living tissues which are composed of immature cells that are capable of division throughout life.
• These tissues are found in growing regions of plants.
• Cells have a thin cell wall.
• Cells contain dense cytoplasm, thin cellulose walls, prominent nuclei, and lack vacuoles.
• Cells contain a prominent and large nucleus.
• Cells are metabolically highly active, so store food is absent.
• Cells are compactly arranged because they do not have intercellular spaces.
• Function: Meristematic tissue is responsible for the growth in length and width(girth) of the plant body.

Classification of Meristematic Tissue

Meristematic Tissue

On the basis of their location meristematic tissues are of three types:

1. Apical Meristem 

• It is present at the growing tips of stems and roots.
• They are responsible for an increase in the length of plant organs.
• Apical meristem increases the length of the stem and root, while intercalary meristem is located near the node in some plants.

Competition Window:

Meristematic tissues are of three types on the basis of origin:

• Promeristem: These are embryonic meristem which give rise to primary meristem.
• Primary Meristem: These are always in active state of division and give rise to primary permanent tissue.
• Secondary Meristem: These are developed from primary permanent tissue and give rise to secondary permanent tissues.

Do You Know?

• Shoot apical meristem is terminal in position whereas Root apical meristem is subterminal in position due to the presence of root cap.
• Apical meristem has two regions at the embryonic stage:
  (i) Promeristem
  (ii) Eumeristem
• Eumeristem is further divided into three regions:
  (a) Protoderm
  (b) Procambium
  (c) Ground meristem

2. Intercalary Meristem 

• It is the part of apical meristem which is left behind during the growth period. They are short-lived and convert into permanent tissue.
• These are present at the base of the leaf or internode. Intercalary meristem seen in some plants is located near the node.
• They are responsible for the growth in the length of plant organs.

3. Lateral Meristem

• It lies on the lateral sides of the stem and root or occurs along the sides of the longitudinal axis of the plant. 
• It helps in increasing the diameter (girth or width) of the plant. Hence helps in secondary growth.
• The girth of the stem or root increases due to lateral meristem (cambium).

Competition Window:

Lateral meristem is both primary and secondary in origin:
(i) Primary lateral meristem
Example: Marginal meristem of leaf and Intra fascicular cambium.
(ii) Secondary lateral meristem
Example: Interfascicular cambium and cork cambium.

Try yourself:Girth of stem increases due to

• A.apical meristem
• B.lateral meristem
• C.intercalary meristem
• D.vertical meristem

Permanent Tissue 

Permanent Tissue

• They are formed by the division and differentiation of meristematic tissue.
• They are composed of those cells which have lost the power of division (temporarily or permanent) and attain a permanent shape, size and function. Cells may be living or dead.
• Cells may be oval, rounded, polygonal or elongated. 
• Permanent tissues are of two types:
  (i) Simple permanent tissue
  (ii) Compound or Complex Permanent Tissue

Simple Permanent Tissue

• These tissues are made up of similar types of cells, that perform a common function. 
• They are protective and supportive in nature.
• Simple tissues are of three types:
  (i) Parenchyma
  (ii) Collenchyma
  (iii) Sclerenchyma

1. Parenchyma

Parenchyma

• It is a living and basic packing tissue which consists of relatively unspecialised cells.
• Cells of these tissues have a thin cell wall that is made up of cellulose.
• Parenchyma consists of living, relatively unspecialised cells with thin cell walls and large intercellular spaces.
• They are usually loosely packed because intercellular spaces are present between cells.
• Functions: Storage of food and provide support to the plant.

Do You Know?

• Parenchyma is the first evolved permanent tissue that is present in all soft parts of the plant (therefore known as universal tissue). The body of bryophytes is made up of only parenchyma tissue.
• In the dorsiventral leaf of the dicot plant, there are two types of parenchyma:
  (i) Spongy Tissue 
  (ii) Palisade Tissue
• Parenchyma generally stores food and may perform photosynthesis or provide support. 
• Prosenchyma: Parenchymatous cells become long and taper at both ends.
  Example: It is found in the pericycle of the root.
• Stellate parenchyma found in the leaf bases of banana, leaf bases performs the function of the stem.
• Differentiation: The development process in which cells take up a permanent shape, size and perform a specific function.

Modification of parenchyma:

(a) Chlorenchyma 

• Such a type of parenchyma in which an abundant quantity of chloroplasts is found.
• They are present in the mesophyll of leaves.
• Function: Synthesis of food (Photosynthesis)

(b) Aerenchyma 

• Such a type of parenchyma is made up of rounded cells which surrounds the large air cavities. It is found in aquatic plants or hydrophytes. 
• Example: Petiole of water hyacinth.
• Function: It provides buoyancy to the aquatic plants to help them float.

2. Collenchyma (Flexible tissue) 

Collenchyma

• Cells of this tissue are living, elongated and irregularly thickened at the corners.
• It is present below the epidermis of the leaf and herbaceous stem in the form of the hypodermis. 
• Intercellular spaces are very little or absent between cells of this tissue. 
• Collenchyma allows bending of various parts of a plant like tendrils and stems of climbers without breaking. 
• Cells of collenchyma contain few chloroplasts.
• Functions
  (i) It provides mechanical support (tensile strength) and elasticity.
  (ii) It protects the cracking of the lamina margin due to the action of wind.
  (iii) It provides flexibility to the plant.
  (iv) It allows easy bending in various parts of the plant (leaf and stem) without breaking.

3. Sclerenchyma

Sclerenchyma

• Sclerenchyma cells are dead cells and they are devoid of protoplasm.
• The walls of cells of sclerenchyma are greatly thickened with the deposition of lignin. Such cell walls are called lignified.
• Due to excessive thickening of the wall of a sclerenchymatous cell, its cavity or lumen becomes nearly absent. The cells of sclerenchyma are closely packed without intercellular spaces.
• They are found in stems (around the vascular bundle), roots, veins of leaves, hard coverings of seed and nuts.
• Function: It is the main mechanical tissue that provides mechanical support.

Sclerenchymatous cells are of two types in structure:

(a) Sclerenchyma Fibres 

• Sclerenchyma cells are long and narrow with thick lignified walls.
• The fibres are closely packed without intercellular spaces. 
• The walls of fibres are often so thick that the cell cavity or lumen is nearly absent. Often oblique thin areas are found in their walls. These are called pits.
• Functions
  (i) Sclerenchyma fibres constitute the major mechanical tissue of the plants and are abundantly found in plants.
  (ii) Commercial fibres obtained from plants (e.g. Jute, Flax, Hemp, Husk of coconut etc.) usually are sclerenchymatous fibres.

Competition Window:

• Lignin is a complex polymer that acts as cement and hardens the cell wall. 
• Lignin makes the cell wall impermeable so important substances are unable to pass through it.
•  As a result, cells that are heavily lignified do not have living content (protoplasm). 

(b) Sclereids (grit or stone cells)

• They are highly thickened and irregularly shaped dead cells. 
• They are found in various parts of the plant such as cortex, pith, phloem, hard seeds etc. 
• The grit of pulp of some fruits (such as guava, apple, pear etc.) is due to the presence of sclereids in them.
• Functions:
  (i) The main function of sclerenchyma is to provide mechanical support to the plants.
  (ii) Sclereids provide strength to seed covering and grittiness to the pulp of many fruits.

Differences Between Fibres and Sclereids

Compound or Complex Permanent Tissue 

• The complex tissues consist of more than one type of cells. All these cells coordinate to perform a common function. 
• Complex tissues transport water, mineral salts (nutrients) and food material to various parts of the plant body. 
• Complex tissues are of the following two types:
  (i) Xylem or wood
  (ii) Phloem 

1. Xylem or Wood

Structure of Xylem Cells

Xylem is made up of four types of cells:

(a) Tracheids 

• Tracheids are elongated cells with tapering ends. 
• They also conduct water. Since tracheids do not have open ends like the vessels, so the water has to pass from cell to cell via the pits.

(b) Vessels or Tracheae 

• Very long tube-like structures formed by a row of cells placed end to end. 
• The transverse walls between the vessels are completely dissolved to form continuous channels or water-pipes.
• Functions
  (i) Tracheids and vessels help in the long-distance conduction of water and minerals upward from the root system to various parts of the plant.
  (ii) Tracheids and vessels provide mechanical support.

(c) Xylem Fibre 

• These are dead and lignified sclerenchymatous cells which are mainly supportive in function.

(d) Xylem Parenchyma

• It is formed of living parenchymatous cells which help in the storage of food and lateral conduction of water and minerals.

Do You Know?

• Xylem and phloem are both conducting tissues and also known as vascular tissues (conducting tissue), together both of them constitute vascular bundles.
• Vessels and tracheids have 5 type of lignification: Annular, spiral, reticulate, scalariform and pitted.
• Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres. Vessels are tubular structures formed by cells placed end to end and help in transport of water and minerals.

Pit Formations  

• No lignin is laid down where plasmodesmata were present in the original cell walls. 
• These non-lignified areas are known as pits and they allow water to pass sideways between one xylem vessel and the next. 
• As vessels and tracheids of xylem have the lignified cell walls, so this simply means that these cells are hollow and there are no cell contents to restrict the flow of water.

Hadrome 

• Tracheids & vessels are collectively known as hadrome and it acts as main conducting elements in the xylem. 
• The annual rings present in the trunk of a tree are xylem rings. By counting the number of annual rings we can determine the age of a tree (Dendrochronology).

2. Phloem

• Function: Phloem transport photosynthetically prepared food materials from the leaves to the storage organs and latter from storage organs to the growing regions of the plant body.
  Phloem

It is made up of four types of cells:

(a) Sieve Tubes  

• Sieve tubes are slender, tube-like structures composed of elongated thin-walled cells, placed end to end. 
• Their end walls are perforated by numerous pores and are called sieve plates. Walls of sieve tubes are perforated. 
• The nucleus of each sieve cell degenerates at maturity, however, cytoplasm persists in the mature cell. Thus, nuclei are absent in mature sieve tube elements. 
• The cytoplasm of one sieve tube element is continuous with those of the sieve elements above and below by cytoplasmic connections passing through the pores of the sieve plate.

Competition Window:

• Although sieve tube elements do not have nuclei, but they still remain living. It is so because they are dependent on adjacent companion cells which develop from the same original meristematic cell. The two cells together for a functional unit.

(b) Companion Cells 

• These are associated with sieve tubes. These are smaller cells having dense cytoplasm and prominent nucleus.
• The companion cells help the sieve tubes in the conduction of food material. 
• Sieve cells & companion cells are also called sister cell because they originate from a single mother cell.

(c) Phloem Parenchyma 

• These are living and thin-walled cells. It is also known as bast parenchyma.
• It helps in the conduction of food in the radial direction. 
• It stores various materials.
  Example: Resin, Latex, Mucilage.

(d) Phloem Fibres  

• These are dead and sclerenchymatous cells. Phloem or bast fibres of some plants are a source of commercial fibres.
  Example: Jute, Hemp, Flax. 
• They provide mechanical support to the conducting elements.

Try yourself:Find out incorrect sentence.

• A.Parenchymatous tissues have intercellular spaces.
• B.Collenchymatous tissues are irregularly thickened at corners.
• C.Apical and intercalary meristems are permanent tissues.
• D.Meristematic tissues, in its early stage, lack vacuoles.

Protective Tissue  

• These tissues are primarily protective in function.
• They are of two types:
  (i) Epidermis
  (ii) Cork

1. Epidermis

• It is the outermost layer of all organs of the plant body which is formed from parenchymal cells. It protects the internal tissue from mechanical injuries and entry of germs.

Epidermis

Cuticle

• The outer wall of the epidermis of aerial parts of the plant is deposited with a fatty substance, called cutin which forms a waterproof layer called the cuticle. 
• It checks the loss of water by transpiration. 
• The lower epidermis of dicot leaves has a large number of microscopic aperture called stomata.

Stomatum 

• Each stomatum is an elliptical aperture bounded by two kidney-shaped guard cells which regulate the opening and closing stomatum. 
• Guard cells are kidney-shaped in dicot and dumb-bell shaped in monocot. 
• Stomata helps in the exchange of gases. 
• It helps in the loss of water vapours called transpiration, develops a force called transpiration pull, which helps in the absorption of water by the roots.

Root Hairs  

• The epidermis of roots (epiblema) has root hairs which greatly increase their surface area for absorption of water and minerals.  

2. Cork or Phellem 

• Cork is the peripheral tissue of old stems and roots of woody trees and is formed due to the activity of cork cambium or phellogen (secondary lateral meristem). 
• Cork cambium produces off new cell on its both sides, thus, forming cork (phellem) on the outer side and the Secondary cortex or phelloderm on the inner side. 
• It is made up of dead cells with a thick wall but no intercellular spaces. 
• The walls of cork cells are heavily thickened by the deposition of an organic substance (a fatty substance), called suberin. 
• Suberin makes these cell impermeable to water and gases and it also helps in the conservation of water in the trees.

Commercial Importance of Cork 

• Cork is light and highly compressible which does not catch fire easily. 
• Cork is used in the making of a variety of sports goods such as cricket balls, table tennis, shuttle-cocks, wooden paddles etc.

Functions

• Cork is protective in function.
• Cork prevents desiccation (by preventing loss of water from the plant).
• Cork prevents infection and mechanical injury.
• Lenticels (pores) present in the cork provide aeration to the inner tissues.

Competition Window:

• Dedifferentiation:  In this process, the specialized cells regain the division power and become meristematic.
  Example: Vascular Cambium
• The cork cells do not contain protoplasm but are filled with resin or tannins. In the case of an onion bulb too, in the skin of onion the cell walls become thick and waterproof due to the addition of suberin.
• Cork and bark are not the same structures.
  While cork includes outer products of cork cambium, the bark includes the outer products of cambium such as secondary phloem and also cork cambium and cork.
• Commercial cork is obtained from the stem surface of Quercus suber.

Chapter Notes: The Fundamental Unit of Life

Introduction

 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.

Materials Required: Onion bulb, Forceps,  Watch glass, Water, Glass slide, Safranin solution, Cover slip, Mounting needle, Thin camel hair paintbrush (optional).

Procedure:

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. 

MULTIPLE CHOICE QUESTION

Try yourself: Who discovered the cell?View Solution

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.

MULTIPLE CHOICE QUESTION

Try yourself: What is the function of the plasma membrane in a cell?View Solution

(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.

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.

MULTIPLE CHOICE QUESTION

Try yourself: What do chromosomes carry information for?View Solution

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.

MULTIPLE CHOICE QUESTION

Try yourself: What is the function of cell organelles?View Solution

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 and Some 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