NCERT Exemplar Class 9 Science Chapter 9 Force and Laws of Motion Solutions
NCERT Exemplar Solutions for Class 9 Science Chapter 9 Force and Laws of Motion covers all the important questions and answers as well as advanced level questions. It helps in learning about the balanced and unbalanced forces, Newton's laws of motion i.e. first law of motion, second law of motion and third law of motion.
The NCERT Exemplar solutions for class 9 science is very important in the examination. NCERT Exemplar Solutions for Class 9 Science Chapter 9 Force and Laws of Motion is provided by our experts. They prepared the best solutions which help the students in understanding the solutions in an easy way. This chapters also covers the other topics like inertia, action and reaction forces, momentum and conversion of momentum.
Chapter Name  Chapter 9 Force and Laws of Motion 
Book Title  NCERT Exemplar for Class 9 Science 
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NCERT Exemplar Solutions for Chapter 9 Force and Laws of Motion Class 9 Science
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.
Solution
(c) It always goes away from the Earth.
Object’s acceleration should be more than the acceleration due to gravity in order to move away from the earth. Only moving along a straight path is not enough to escape from gravity.
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.
Newton’s third law of motion states that “For every action there is equal and opposite reaction”.
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.
Pulling hands backwards help the goalkeeper to reduce the momentum of the ball. This in turn reduces the force of the ball exerted on goalkeeper’s 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
Inertia is the property which resists any change in the state of motion of the object. Object remains in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.
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
The coin would have fallen in the passenger’s hand if the motion of train was uniform. If the motion of the train would have been retarded then the coin would have fallen ahead of him. As the coin falls behind the passenger, the motion of the train is an accelerated one.
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
The friction is opposing the force. Thus no force is required to keep the object in uniform motion.
7. Rocket works on the principle of conservation of :
(a) Mass
(b) Energy
(c) Momentum
(d) Velocity
Solution
(c) Momentum
Velocity of hot gases coming out of rocket provided large momentum in opposite direction. This makes the rocket move upwards. Here the conservation of momentum takes place.
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
The water tanker comes to rest on applying the brakes but water will be in motion. This makes the water to come forward.
Short Answer Questions
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. When the brakes are applied, train will come to rest and the balls try to attain rest. Now, because of inertia, the balls remain in motion and they continue to roll. The mass of two balls is not the same, thus the inertia of iron ball is greater than the inertia of the rubber ball. This is the reason why the rubber ball rolls faster than the iron 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
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, the horse should overcome the frictional force. Therefore it continues to apply the force.
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
F = ma
⇒ F = 0.5 N
15. A 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
Force = mass ×acceleration
F = ma
a = F/m
When mass is doubled, force is halved, m becomes 2m and F becomes F/2.
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 sprinkler which rotates as soon as water is supplied to it is based on the thirdlaw of motion. As the water comes out of the nozzle of the sprinkler, an equal and opposite reaction force comes into play which rotates the sprinkler.
Long Answer Questions
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
(ii) v = u + at
⇒ 0 = 10^{3}  10^{7} t
⇒ 10^{7} t = 10^{3}
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
F = m a = kg m s^{–2}
This unit is also called newton. Its symbol is N.
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 is the quantity of motion of a moving body. It is measured as product of mass and velocity.
Momentum = mass × velocity
SI unit of momentum is kg m s–1
Force = Rate of change in momentum
(a) Mass is fixed