NCERT Exemplar Solutions for Class 9 Science Chapter 9 Force and Laws of Motion (MCQ, SAQ and LAQ)
Chapter Name  NCERT Exemplar Solutions for Class 9 Science Ch 9 Force and Laws of Motion 
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Objective Type Questions for Force and Laws of Motion
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.
Solution
(c) It always goes away from the Earth.
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.
Solution
(b) Always act on different bodies 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 goalkeeper 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.
Solution
(b) Reduce the force exerted by the ball on 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
Solution
(c) To resist any change in its state of motion
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
Solution
(a) Accelerated
6. An object of mass 2 kg is sliding with a constant velocity of 4 ms1 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
Solution
(b) 0 N
7. Rocket works on the principle of conservation of :
(a) Mass
(b) Energy
(c) Momentum
(d) Velocity
Solution
(c) Momentum
8. A water tanker filled up to 2/3 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
Solution
(b) Move forward
Short Answer Questions Force and Laws of Motion
9. There are three solids made up of aluminum, steel and wood, of the same shape and same volume. Which of them would have highest inertia ?
Solution
As the mass is a measure of inertia, the ball of same shape and size, having more mass than other balls will have highest inertia. Since steel has greatest density and greatest mass, therefore, it has 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.
Solution
Yes, both the balls will start rolling due to inertia of and they roll in the direction in which the train was moving. Due to the application of the brakes, the train comes to rest but due to inertia the balls try to remain in motion, therefore, they continue to roll. The two balls will not roll with the same speed. Since the masses of the balls are not the same, therefore, the inertial forces are also not same on both the balls. Thus, the balls will move with different speeds. In fact the iron being heavier will move with a speed comparatively lesser than that of the rubber ball.
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?
Solution
As both the bullets are said to be identical and are fired with the same force, therefore, as per Newton’s third law of motion, ‘Every action has an equal and opposite reaction’; same force will be applied on both the rifles. As the same amount force is applied on both the rifles, the lighter rifle will move more quickly in the backward direction causing serious injury to the shoulder.
12. A horse continues to apply a force in order to move a cart with a constant speed. Explain, why?
Solution
A horse continues to apply a force to move a cart with a constant speed so as to overcome the force of friction between the cart and the ground. Once the cart starts moving, the force of friction comes into action. It starts working on the wheels of the cart in a direction opposite to the direction of motion of the cart. So, the horse has to apply a continuous force in the forward direction to maintain 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 is same. Yet, it is not an example of conservation of momentum. Explain, why?
Solution
Law of conservation of momentum is applicable to isolated system. An isolated system is a system that is free from the influence of a net external force that alters the momentum of the system. In the given example, the change in velocity is due to the gravitational force of Earth. This is an external force. Therefore, it is not an example of conservation of momentum.
14. Velocity versus time graph of a ball of mass 50 g rolling on a concrete floor is shown in figure. Calculate the acceleration and frictional force of the floor on the ball.
Solution
Given, m = 50 g, F = ?
The velocity of the ball zero time is 80 ms^{1} . It decelerates due to the friction of the floor with itself and comes to rest after 8 s
a = change in velocity/time
= [(0 – 80)/8 ]ms^{2} = 10 ms^{2}
The negative sign indicates that the frictional force exerted opposes the motion of the ball. Now, using Newton’s relation
F = ma
F = 50/1000 kg × ( 10) ms^{2}
F = 0.5 N
15. 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?
Solution
Given, the initial mass m_{1}= M, initial force f_{1}= F
Given that, new mass m_{2} = M + M = 2M and new
force f_{2} = F/2 (as force is halved)
From Newton’s second law, F = ma
16. Two friends on rollerskates 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.
Solution
Separation between them will increase. Initially the momentum of both of them are zero. In order to conserve the momentum the one who throws the ball would move backward. The second will experience a net force after catching the ball and therefore, will move backwards that is in the direction of the force.
17. Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.
Solution
The working of the rotation of sprinkler is based on third law of motion. As the water comes out of the nozzle of the sprinkler, an equal and opposite reaction force comes into play. So, the sprinkler starts rotating.
18. Using second law of motion, derive the relation between force and acceleration. A bullet of 10 g strikes a sandbag at a speed of 103 ms^{–1} 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.
Solution
19. Derive the unit of force using the second law of motion. A force of 5 N produces an acceleration of 8 ms^{–2} on a mass m_{1} and an acceleration of 24 ms^{–2} on a mass m_{2}. What acceleration would the same force provide if both the masses are tied together?
Solution
According to Newton’s second law of motion,
Force = mass × acceleration
Unit of mass = kg
Unit of acceleration = m/s^{2}
F = Kg × m/s^{2}
Therefore, unit of force = kg.m/s^{2}
1 kgm/s^{2} = 1 Newton
Given, F = 5 N, mass = m_{1} , a = 8 ms^{2}
F = ma
5 = m_{1} × 8
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.
Solution
Momentum = mass × velocity
SI unit of momentum is kg ms^{–1}
Force = Rate of change in momentum