Q1: What is force?
Ans: Force is a physical quantity that can make an object move from rest, change its speed, change its direction of motion, or change its shape.
Q2: What is the SI unit of force and what is its symbol?
Ans: The SI unit of force is newton, written with a small ‘n’, and its symbol is N.
Q3: Name the instrument used to measure the magnitude of a force.
Ans: A spring balance is used to measure the magnitude of a force.
Q4: What are balanced forces?
Ans: Two forces that are equal in magnitude but opposite in direction are called balanced forces. They do not cause any change in the motion of an object.
Q5: What is net force?
Ans: When two or more forces act on an object and are not balanced, the resultant force acting on the object is called the net force.
Q6: In which direction does the force of friction act on a moving object?
Ans: The force of friction always acts on a moving object in the direction opposite to its direction of motion.
Q7: State Newton’s first law of motion.
Ans: An object at rest remains at rest and an object in motion continues to move with a constant velocity, unless a net force acts upon the object.
Q8: Who introduced the word ‘inertia’ and what does it mean?
Ans: Isaac Newton introduced the word ‘inertia’ to describe the tendency of objects to resist any change in their state of rest or uniform motion.
Q9: What is the acceleration of an object when no net force acts on it?
Ans: When no net force acts on an object, its acceleration is zero. The object either remains at rest or continues moving with a constant velocity.
Q10: State Newton’s second law of motion.
Ans: When a net force acts on an object, the object accelerates in the direction of the net force. The acceleration is proportional to the net force and inversely proportional to the mass of the object.
Q11: Write the mathematical formula that represents Newton’s second law of motion.
Ans: F = ma, where F is the net force, m is the mass of the object, and a is the acceleration produced.
Q12: Define one newton of force.
Ans: One newton is defined as the force that produces an acceleration of 1 m s⁻² on an object of mass 1 kg.
Q13: What is the value of acceleration due to gravity near the surface of the Earth?
Ans: The value of acceleration due to gravity near the surface of the Earth is g = 9.8 m s⁻², which can be taken as approximately 10 m s⁻² for quick estimations.
Q14: Why does a fielder pull their hands backwards while catching a fast-moving cricket ball?
Ans: By pulling the hands backwards, the time over which the ball’s velocity reduces to zero increases. This reduces the acceleration and therefore the force exerted on the hands, minimising injury.
Q15: State Newton’s third law of motion.
Ans: Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object.
Q16: Do the equal and opposite forces in Newton’s third law act on the same object?
Ans: No, the pair of equal and opposite forces in Newton’s third law always act on two different objects, so they do not cancel each other.
Q17: How does a rocket lift off from the ground?
Ans: A rocket’s engine expels gases downwards. By Newton’s third law, the gases exert an equal and opposite force on the rocket in the upward direction, causing it to lift off.
Q18: What is the normal force acting on an object placed on a surface?
Ans: The normal force is the force exerted by a surface on an object placed on it, acting perpendicular to the surface in the upward direction. It balances the weight of the object.
Q19: What are internal and external forces in a system of objects?
Ans: Forces acting between objects within a system are called internal forces, while forces acting on the system from outside are called external forces. Only external forces affect the motion of the system.
Q20: Why is it difficult to walk on a wet polished floor or ice?
Ans: On wet polished floors or ice, the force of friction between the feet and the surface is very low. Since friction is what pushes a person forward while walking, less friction makes it difficult to walk without slipping.