NCERT Solutions for Class 9 Science Chapter 8 – Force and Laws of Motion

Force and Laws of Motion form the foundation of classical mechanics and help explain how objects move or remain at rest. A force is a push or pull that can change the state of motion, shape, or direction of an object.

This chapter introduces Newton’s three laws of motion, which describe the relationship between force, mass, and acceleration. It also explains concepts like inertia, momentum, and conservation of momentum, which are essential for understanding real-life phenomena such as motion of vehicles, collisions, and everyday activities.

NCERT Solutions for Class 9 Science Chapter 8 – Force and Laws of Motion

Force and Laws of Motion
Easy
Q.
An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason.
Force and Laws of Motion
Easy
Q.
Two objects, each of mass 1.5 kg, are moving in the same straight line but in opposite directions. The velocity of each object is 2.5 ms-1 before the collision during which they stick together. What will be the velocity of the combined object after collision?
Force and Laws of Motion
Easy
Q.
Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.
Force and Laws of Motion
Medium
Q.
An object of mass 100 kg is accelerated uniformly from a velocity of 5 ms-1 to 8 ms-1 in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.
Force and Laws of Motion
Medium
Q.
An object of mass 1 kg travelling in a straight line with a velocity of 10 ms-1 collides with, and sticks to, a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object.
Force and Laws of Motion
Easy
Q.
A bullet of mass 10 g travelling horizontally with a velocity of 150 ms−1 strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.
Force and Laws of Motion
Easy
Q.
A hockey ball of mass 200 g travelling at 10 ms−1 is struck by a hockey stick so as to return it along its original path with a velocity at 5 ms−1. Calculate the magnitude of change of
momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.
Force and Laws of Motion
Easy
Q.
According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
Force and Laws of Motion
Easy
Q.
Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?
Force and Laws of Motion
Easy
Q.
When a carpet is beaten with a stick, dust comes out of it. Explain.
Force and Laws of Motion
Easy
Q.
What is the momentum of an object of mass m, moving with a velocity v?
  1. (mv)2
  2. mv2
  3. ½ mv2
  4. mv
Force and Laws of Motion
Easy
Q.
An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7 ms−2?
Force and Laws of Motion
Medium
Q.
A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate:

(a) the net accelerating force and

(b) the acceleration of the train.
Force and Laws of Motion
Easy
Q.
A stone of 1 kg is thrown with a velocity of 20 ms−1 across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. What is the force of friction between the stone and the ice?
Force and Laws of Motion
Easy
Q.
A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if its mass is 7 tonnes (Hint: 1 tonne = 1000 kg.)
Force and Laws of Motion
Medium
Q.
A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because
(a)  the batsman did not hit the ball hard enough.
(b)  velocity is proportional to the force exerted on the ball.
(c)   there is a force on the ball opposing the motion.
(d)   there is no unbalanced force on the ball, so the ball would want to come to rest.
Force and Laws of Motion
Easy
Q.
Why is it advised to tie any luggage kept on the roof of a bus with a rope?
Force and Laws of Motion
Medium
Q.
How much momentum will a dumb-bell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration to be 10 m s−2.

NCERT Solutions for Class 9 Science Chapter 8 – Force and Laws of Motion

NCERT Solutions for Class 9 Science Chapter 8 – Force and Laws of Motion (Q&A)

Q1. Can an object have non-zero velocity even when the net external force is zero?

Ans: Yes, it is possible. An object can move with a non-zero velocity if it is moving with a constant velocity in a straight line. No force is required to maintain uniform motion, but a force is needed to change it.

Q2. Why does dust come out of a carpet when it is beaten?

Ans: Due to inertia, dust particles tend to remain at rest. When the carpet is beaten, it moves suddenly, but the dust particles resist this change and get separated from the carpet.

Q3. Why should luggage on the roof of a bus be tied?

Ans: When the bus starts suddenly, the luggage tends to remain at rest due to inertia and may fall backward. Tying it prevents it from falling off.

Q4. Why does a cricket ball come to rest after rolling on the ground?

Ans:
Correct option: (c) There is a force on the ball opposing the motion.

The ball slows down due to frictional force acting opposite to its direction of motion.

Q5. A force of 200 N is applied to move a cabinet at constant velocity. What is the frictional force?

Ans:
The frictional force is 200 N, equal and opposite to the applied force, since the object moves with constant velocity.

Q6. Why does a truck not move when pushed, despite equal and opposite forces?

Ans:
The logic is incorrect. The truck does not move because the static friction between the truck and the road balances the applied force. Due to its large mass, friction is very high, preventing motion.

Q7. In a collision between a car and an insect, whose explanation is correct?

Ans:

  • Kiran is correct: The insect undergoes a larger change in velocity, hence larger effect.
  • Akhtar is correct: The car exerts a large force due to its mass and speed.
  • Rahul is partially correct: Both experience equal and opposite forces (Newton’s third law), but total momentum remains conserved.

FAQs

Q1. What is force?

Force is a push or pull that can change the state of motion or shape of an object.

Q2. What is inertia?

Inertia is the tendency of an object to resist any change in its state of rest or motion.

Q3. What are Newton’s laws of motion?

  • First Law: Law of inertia
  • Second Law: Force = mass × acceleration
  • Third Law: Every action has an equal and opposite reaction

Q4. What is momentum?

Momentum is the product of mass and velocity of an object.

Q5. What is the law of conservation of momentum?

The total momentum of a system remains constant if no external force acts on it.

Q6. What is friction?

Friction is a force that opposes the motion of an object.

Q7. Why is friction important?

Friction helps in walking, gripping objects, and stopping moving vehicles.