Important Questions for Class 9 Chapter 9 - Force and Laws of Motion

Important questions based on NCERT syllabus for Chapter 9 - Force and Laws of Motion:

Question-1: A boy standing in a bus is thrown towards the rear of the bus as it begins to move, and to the front when it slows down. Why does this happen?

Solution: 1. When the boy is standing in a bus and the bus begins to move suddenly, he tends to fall backwards. This is because the sudden start of the bus sets the busin motion including his feet in contact which is in with the floor of the bus. But the rest of his body opposes the motion because of inertia, hence the bus moves forward and the boy is thrown to the rear of the bus.

  1. When the bus slows down, the boy is still in motion and his body continues to move in the same direction, hence he tends to move forward.

Question-2: Why a person is likely to get more hurt when he falls on concrete floor than on loose mud or sand ?

Solution: When a person falls on the ground with a concrete or a mud floor, he exerts a force on the ground. According to Newton’s Third law of motion, the ground also exerts an equal but opposite force on the person. The person is likely to get more hurt when he hits on the concrete floor because his impact time will be less, hence the force will be greater.

Impulse = accelerated force x time

In a mud ground, the contact time will be more, hence the force exerted will be less. Hence the impact is less.

Question-3: Name and state the action and reaction in the following cases :
(i) Firing a bullet from a gun.
(ii) Hammering a nail.
(iii) A book lying on a table.
(iv) A moving rocket.
(v) A person walking on the floor.

Solution: (i) Force on bullet is action, force on gun is reaction.
(ii) Force on hammer is reaction and force on nail is action
(iii) Force on the table by gravity is action and force on book by table is reaction.
(iv) Force on gases coming out is action, force on rocket is reaction.
(v) Force by person on the ground is action and force by ground on the person is reaction.