Concepts

Class 9
Chapter 9 Class 9 - Force and Laws Of Motion

## What is inertia?

It is a property of body by which it resists change

This change may be

change in state of rest

or

change in state of motion

## Different Examples of Inertia

When car starts suddenly, passengers move backwards

When a car starts suddenly, the lower part of the body which is in contact with the car is in motion. But the upper part is still at rest due to inertia. Hence, the passenger moves backwards

When car applies brakes,passengers get pushed forward

When a car stops suddenly, the lower part of the body which is in contact with the car stops. But the upper part is still in motion due to inertia. Hence, the passenger is pushed forward

When hanging carpet is beaten with stick,dust fall off

When the carpet is beaten, it comes in motion. But the dust particles remain at rest due to inertia. Thus, dust falls off due to the force of gravity

When a tree is shaken vigorously, its fruits and leaves fall down.

When a tree is shaken, it comes in motion. But the fruits and leaves remain at rest due to inertia. Thus, they fall off due to the force of gravity

When a car/bus takes a sharp turn, we tend to fall sideways.

When a car takes a sharp turn, the lower part of the body which is in contact with the car also takes the sharp turn. But the upper part is still moving in a straight line due to inertia. Hence, the passenger fall sideways

It is dangerous to jump out of a moving bus.

When we jump from a moving bus, our feet which is in contact with the ground comes to rest. But, our upper body remains in

motion due to inertia. Hence, we may fall on the ground and get hurt

Which objects have more inertia - Heavy or Light?

Heavy objects have more inertia than lighter object

Example 1

It is easier to throw a small stone than a heavy stone

Example 2

If we throw 2 balls,one tennis ball and other cricket ball with equal force

Cricket ball travels lesser distance than tennis ball because it is heavier

We can say that cricket ball has more inertia than tennis ball so it offers more resistance to its change of rest

## Questions

Q 1 Page 118 - Which of the following has more inertia:

(a) a rubber ball and a stone of the same size?

(b) a bicycle and a train?

(c) a five rupees coin and a one-rupee coin?

Q 3 Page 118 - Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Q 4 Page 118 - Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

NCERT Question 2 - When a carpet is beaten with a stick, dust comes out of it. Explain.

NCERT Question 3 - Why is it advised to tie any luggage kept on the roof of a bus with a rope?