NCERT Class 9 Science Solutions: Chapter 9 – Force and Laws of Motion Part 1 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 1

Which of the following has more inertia:

Q. (a) A Rubber ball and a stone of the same size?

Inertia is the measure of the mass of the body. The greater is the mass of the body; the greater is its inertia and vice-versa.

Mass of a stone is more than the mass of a rubber ball for the same size. Hence, inertia of the stone is greater than that of a rubber ball.

Q. (b) A bicycle and a train?

Inertia is the measure of the mass of the body. The greater is the mass of the body; the greater is its inertia and vice-versa.

Mass of a train is more than the mass of a bicycle. Hence, inertia of the train is greater than that of the bicycle.

Q. (c) A five- Rupees coin and a one-rupee coin?

Inertia is the measure of the mass of the body. The greater is the mass of the body; the greater is its inertia and vice-versa.

Mass of a five rupee coin is more than that of a one-rupee coin. Hence, inertia of the five rupee coin is greater than that of the one-rupee coin.

Question 2

Q. In the following example, try to identify the number of times the velocity of the ball changes:

“A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team.”

Also identify the agent supplying the force in each case.

The velocity of the ball changes four times.

As a football player kicks the football, its speed changes from zero to a certain value. As a result, the velocity of the ball gets changed. In this case, the player applied a force to change the velocity of the ball. Another player kicks the ball towards the goal post. As a result, the direction of the ball gets changed. Therefore, its velocity also changes. In this case, the player applied a force to change the velocity of the ball. The goalkeeper collects the ball. In other words, the ball comes to rest. Thus, its speed reduces to zero from a certain value. The velocity of the ball has changed. In this case, the goalkeeper applied an opposite force to stop/change the velocity of the ball. The goalkeeper kicks the ball towards his team players. Hence, the speed of the ball increases from zero to a certain value. Hence, its velocity changes once again. In this case, the goalkeeper applied a force to change the velocity of the ball.

Agent supplying the force –

First case – first player

Second case – second player

Third case – Goalkeeper

Fourth case – Goalkeeper

Question 3

Q. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Some leaves of a tree get detached when we shake its branches vigorously. This is because when the branches of a tree are shaken, it moves to and fro, but its leaves tend to remain at rest. This is because the inertia of the leaves tend to resist the to and fro motion. Due to this reason, the leaves fall down from the tree when shaken vigorously.

Question 4

Q. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Due to the inertia of the passenger.

Everybody tries to maintain its state of motion or state of rest. If a body is at rest, then it tries to remain at rest. If a body is moving, then it tries to remain in motion. In a moving bus, a passenger moves with the bus. As the driver applies brakes, the bus comes to rest. But, the passenger tries to maintain his state of motion. As a result, a forward force is exerted on him. Similarly, the passenger tends to fall back words when the bus accelerates from rest. This is because when the bus accelerates, the inertia of the passenger tends to fall back wards when the bus accelerates forward.

Question 1

Q. If action is always equal to the reaction, explain how a horse can pull a cart.

A horse pushes the ground in the backward direction. According to newton՚s third law of motion, a reaction force is exerted by the earth on the horse in the forward direction. As a result, the cart moves forward.

Question 2

Q. Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.

Due to the backward reaction of the water being ejected.

When a fireman holds a hose, which is ejecting large amounts of water at a high velocity, then a reaction force is exerted on him by the ejecting water in the backward direction. This is because of Newton՚s third law of motion. As a result of the backward force, the stability of the fireman decreases. Hence it is difficult for him to remain stable while holding the hose.

Question 3

Q. From a rifle of mass 4 kg, a bullet of mass 50g is fired with an initial velocity of 35 . Calculate the initial the recoil velocity of the rifle.

Mass of the rifle,

Mass of the bullet,

Recoil velocity of the rifle =

Bullet is fried with an initial velocity,

Initially, the rifle is at rest.

Thus, its initial velocity,

Total initial momentum of the rifle and bullet system

Total momentum, of the rifle and bullet system after firing:

According to the law of conservation of momentum:

Total momentum after the firing Total momentum before the firing

The negative sign indicates that the rifle recoils backward with a velocity of 0.4375 m/s.

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