9. Force and Laws of Motion Class 9 NCERT Exemplar Solutions Chapter 9 (CBSE Science)
CBSE Class 9 Science Chapter 9: Force and Laws of Motion – NCERT Exemplar Questions with Answers
Chapter 9: Force and Laws of Motion
Multiple Choice Questions
1. Which of the following statement is not correct for an object moving along a straight path in an accelerated motion?
(a) Its speed keeps changing (b) Its velocity always changes
(c) It always goes away from the earth (d) A force is always acting on it
Answer: (c) It always goes away from the earth
[ An object moving in accelerated motion along a straight path does not always move away from the Earth. It may move towards the Earth or in any direction. The other statements are correct because accelerated motion means there is a change in velocity, and a force acts on the object.]
2. According to the third law of motion, action and reaction
(a) always act on the same body
(b) always act on different bodies in opposite directions
(c) have same magnitude and directions
(d) act on either body at normal to each other
Answer: (b) always act on different bodies in opposite directions
[ According to Newton’s third law of motion, every action has an equal and opposite reaction. The action and reaction forces act on two different bodies and in opposite directions.]
3. A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goal keeper to
(a) exert larger force on the ball (b) reduce the force exerted by the ball on hands
(c) increase the rate of change of momentum (d) decrease the rate of change of momentum
Answer: (b) reduce the force exerted by the ball on hands
[ By pulling his hands backwards, the goalkeeper increases the time taken to stop the ball. According to the relation between force and change in momentum, increasing the time decreases the force acting on the hands.]
4. The inertia of an object tends to cause the object
(a) to increase its speed (b) to decrease its speed
(c) to resist any change in its state of motion (d) to decelerate due to friction
Answer: (c) to resist any change in its state of motion
[ Inertia is the property of a body by which it opposes any change in its state of rest or uniform motion in a straight line (Newton’s First Law).]
5. A passenger in a moving train tosses a coin which falls behind him. It means that motion of the train is
(a) accelerated (b) uniform (c) retarded (d) along circular tracks
Answer: (a) accelerated
[ If the coin falls behind the passenger, the train must be accelerating forward. Due to inertia, the coin continues with the train’s earlier (lower) speed and lands behind him.]
5. An object of mass 2 kg is sliding with a constant velocity of 4 m/s on a frictionless horizontal table. The force required to keep the object moving with the same velocity is
(a) 32 N (b) 0 N (c) 2 N (d) 8 N
Answer: (b) 0 N
[ Since the object is moving with constant velocity, its acceleration is zero.
Here, m = 2 kg , a = 0
From Newton’s Second Law, 
On a frictionless surface, no force is needed to maintain uniform motion.]
7. Rocket works on the principle of conservation of
(a) mass (b) energy (c) momentum (d) velocity
Answer: (c) momentum
[ A rocket moves forward by ejecting gases backward at high speed.
According to the law of conservation of momentum, the backward momentum of gases equals the forward momentum of the rocket. ]
8. A water tanker filled up to 
of its height is moving with a uniform speed. On sudden application of the brake, the water in the tank would
(a) move backward (b) move forward (c) be unaffected (d) rise upwards
Answer: (b) move forward
[ When the brakes are applied suddenly, the tanker stops but the water continues to move forward due to inertia of motion. Therefore, the water in the tank moves towards the front side.]
Short Answer Questions
9. There are three solids made up of aluminium, steel and wood, of the same shape and same volume. Which of them would have highest inertia?
Answer: The steel solid would have the highest inertia because inertia depends on mass. For the same shape and volume, steel has the greatest density and therefore the greatest mass among aluminium, steel and wood. Hence, steel has the highest inertia.
10. Two balls of the same size but of different materials, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer.
Answer: Yes, both balls will start rolling in the forward direction when the brakes are applied suddenly. This happens due to inertia of motion. They will not move with the same speed because iron and rubber have different masses and frictional effects, so their motion will be different.
11. Two identical bullets are fired one by a light rifle and another by a heavy rifle with the same force. Which rifle will hurt the shoulder more and why?
Answer: The light rifle will hurt the shoulder more because for the same force the lighter rifle recoils with a larger acceleration and gives a greater jerk to the shoulder.
12. A horse continues to apply a force in order to move a cart with a constant speed. Explain why?
Answer: The horse must apply a force to balance the friction and air resistance acting on the cart. When this applied force equals the opposing forces the net force becomes zero and the cart moves with constant speed.
13. Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of initial and final momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why ?
Answer: The law of conservation of momentum applies only when no external force acts. Here gravity acts as an external force on the ball throughout its motion. So although the initial and final momenta are equal in magnitude momentum is not conserved because the external force changes its direction.
14. Velocity versus time graph of a ball of mass 50 g rolling on a concrete floor is shown in Fig. 9.1. Calculate the acceleration and frictional force of the floor on the ball.
Solution: Here, Initial velocity, 
, Final velocity, 
, Time taken, 
, Mass ,
We have, 
Therefore, the acceleration of the ball is −10 m/s² and the frictional force acting on it is −0.5 N.
15. A truck of mass M is moved under a force F. If the truck is then loaded with an object equal to the mass of the truck and the driving force is halved, then how does the acceleration change?
Answer: Given, the initial mass of the truck be M and force applied be F.
We have, 
Initial acceleration, 
After loading: total mass 
and 
New acceleration,
So, the new acceleration becomes one-fourth of the original acceleration.
16. Two friends on roller-skates are standing 5 m apart facing each other. One of them throws a ball of 2 kg towards the other, who catches it, How will this activity affect the position of the two? Explain your answer.
Answer: When one friend throws the ball he moves backward due to recoil. The other friend moves backward after catching the ball because the ball exerts force on him. Thus both friends move farther apart as a result of conservation of momentum.
17. Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.
Answer: A water sprinkler works on Newton’s third law of motion. When water comes out with force through the bent pipes the sprinkler experiences an equal and opposite reaction force. This reaction force makes the sprinkler rotate.
Long Answer Questions
18. Using second law of motion, derive the relation between force and acceleration. A bullet of 10 g strikes a sand-bag at a speed of 103 m/s and gets embedded after travelling 5 cm. Calculate
(i) the resistive force exerted by the sand on the bullet
(ii) the time taken by the bullet to come to rest.
Answer: Derivation using Newton’s Second Law:
Newton’s second law states: The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force.
Initial momentum 
and final momentum 
Change in momentum 
Rate of change of momentum 
If 
, then 
Thus, Force is directly proportional to acceleration (for constant mass).
Mass of bullet, m = 10 g = 0.01 kg , Initial speed, u = 10³ m/s = 1000 m/s , Final speed, v = 0 m/s , Distance travelled inside sand, s = 5 cm = 0.05 m
(i) We have, 
m/s²
Resistive force 
N .
(ii) We have,
19. Derive the unit of force using the second law of motion. A force of 5 N produces an acceleration of 8 m/s² on a mass m1 and an acceleration of 24 m/s² on a mass m2 . What acceleration would the same force provide if both the masses are tied together?
Answer: From newton’s Second Law, We have,
Mass (m) = 1 kilogram (kg) and Acceleration (a) = 1 m/s²
Thus, 1 N is the force required to produce 1 m/s² acceleration in a body of mass 1 kg.
Given, 
For mass 
, acceleration 
m/s²
For mass 
, acceleration 
m/s²
New acceleration, 
So, the acceleration of the combined masses is 6 m/s².
20. What is momentum? Write its SI unit. Interpret force in terms of momentum.
Represent the following graphically
(a) momentum versus velocity when mass is fixed.
(b) momentum versus mass when velocity is constant.
Answer: The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. Its SI unit is kg m/s .
(a) We have, 
If mass is constant, then momentum is directly proportional to velocity.
So, the graph will be a straight line passing through the origin.
(b) We have,
If velocity is constant, then momentum is directly proportional to mass.
So, the graph will be a straight line passing through the origin.
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