NCERT Class-9 Science Solutions: Chapter 9-Force and Laws of Motion Part 2 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Short Answer Questions

Question 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?


Steel: 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.

Question 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.


Yes. The balls will start rolling 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 begin to roll. Since the masses of the balls are not the same, therefore, the inertial forces are not same on both the balls. Thus, the balls will move with different speeds.

Balls Start Rolling in the Train Was Moving

Question 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?


From the light rifle, according to law of conservation of momentum or explanation by Newton՚s laws of motion.

As both the bullets are identical and are fired with the same force. Therefore, to reduce recall velocity, we use, heavy rifle so that its heavy mass compensate the large momentum. But for light rifle, recoil velocity will be large due to small mass thus light rifle will hurt more to the shoulder.

Question 12:

A horse continues to apply a force in order to move a cart with a constant speed. Explain why?


The force applied by the horse balances the force of friction. According to Newton՚s First Law a body remains in its state of motion or rest until an external force acts on it.

But in this case the horse has to continuously exert the force to move the cart, this is because the force of friction is acting against the motion of the cart which us opposing its motion. Therefore, the horse has to apply force to overcome this force of friction.

Question 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?


Law of conservation of momentum is applicable to isolated system (no external force is applied) . In this case, the change in velocity is due to the gravitational force of earth. Momentum of a system remains conserved if no external force acts on the system. In the given example, there is gravitational force acting on the ball which is an external force, so it is not an example of conservation of momentum

Question 14:

Velocity versus time graph of a ball of mass rolling on a concrete floor is shown in Figure. Calculate the acceleration and frictional force of the floor on the ball.

Image Velocity Versus Time Graph


(Naegative sign shown against the direction of motion)

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