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Consider you are travelling in a bus or a car, When the brake is applied suddenly, the upper part of your body moves in the forward direction. Likewise, when the vehicle suddenly moves forward from rest, you lean backwards. This is because all bodies would like to remain in their state of rest or the state of motion. This concept is called ‘Inertia’.
The essential property of a body to resist any change in its state of rest or the state of uniform motion, unless influenced by an external unbalanced force, is known as ‘Inertia’.
The inertia of a body depends on its mass. The inertia of a body is directly proportional to its mass, i.e. greater the mass of a body, the greater is its inertia.
Types of inertia
Inertia of rest:

The resistance of a body to change its state of rest is called inertia of rest.
Inertia of motion:
The resistance of a body to change its state of motion is called inertia of motion.
Inertia of direction:
The resistance of a body to change its direction of motion is called inertia of direction.
Examples of Inertia:
  • A Long jump or high jump athlete runs some distance before jumping. By doing so, this will help him jump longer and higher (Inertia of motion).
  • While driving a car, when you make a sharp turn, you tend to lean sideways (Inertia of direction).
  • When you forcefully shake the branches of a tree, some of the leaves and fruits are detached, and they fall down (Inertia of rest).