# Inertia and Mass

Inertia is the property or tendency of a body to resist any change in its state of rest or of uniform motion in a straight line. In other words, the inability of a body to change its state of rest or of uniform motion. The name inertia was given by Galileo. A body is said to have a large inertia if it is hard to change its state of rest or of uniform motion. If we kick a football, it flies away. But if we kick a stone of the same size it does not move at all. An object such as a heavy truck or a heavy stone has a large amount of inertia, whereas a pencil or a safety pin has very little inertia. Thus, mass is a measure of inertia of a body. The more the mass, the greater is its inertia.

If a body has a large inertia, it is hard to make it move. If a body with a large inertia is moving, it is difficult to stop it or change its speed. Similarly, it is hard to change its direction of motion, i.e. it is hard to change its velocity.

Inertia and Mass

Experiment to demonstrate Inertia of Rest :-

Place a 50 paise coin on a cardboard placed on a tumbler as shown in the visual. Now flip the card quickly with a finger. The coin will neatly drop into the tumbler. This experiment illustrates the inertia of rest of the coin.

# Experiment to illustrate Inertias of Rest and Motion

The visual shows a toy car free to move on a smooth steel track. If the track is jerked quickly to the left or right, the wheels of the car will turn but the car itself continues to stay at rest, i.e. the car has inertia of rest. Now, if the car is given a push, it rolls along the track. If the friction between the wheels and the track is negligible and if the track is sufficiently long, the car moves with the same velocity for a considerable period of time. A railway wagon detached from the engine can travel a few kilometres before coming to rest. It will never stop if the friction between the wheels and the track and that offered by air were absent.

Inertia of motion