Theory:

Inertia:
The body's tendency to oppose the change in its state of rest or uniform motion in a straight line is called inertia. Newton's first law of motion is also called the law of inertia.
 
Mass:
The inertia of a body depends upon mass. We know it is not easy to move a heavier object than the lighter one. Similarly, it is not easy to stop a moving heavier body than the moving lighter one. Thus, we can conclude that the mass of  body is the measure of inertia; more the mass of a body, more the inertia.
Example:
If we kick a football, it flies away. But if we kick a stone of the same size with equal force, it hardly
moves.
Types of inertia:
  • Inertia of rest
  • Inertia of motion
  • Inertia of direction
Inertia of rest:
The body's tendency to oppose the change in its state of rest when unbalanced force is applied to it is called the inertia of rest.
 
For example, a person in a bus falls backwards when the bus starts suddenly. This is because the lower part of his body begins to move along with the bus, but the upper part of the body tends to remains at rest due to the inertia of rest.
 
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Inertia of Motion:
The body's tendency to oppose the change in its state of motion when unbalanced force is applied to it is called the inertia of motion.
 
For example, a man travelling in a bus falls forward when the driver applies the brake, it is because his feet comes to rest suddenly, whereas the upper part of his body retains the forward motion.
 
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Inertia of Direction:
The tendency of the body to oppose the change in its direction of motion is called the inertia of direction.
 
For example: Tie a stone to one end of a string, by holding the other end of the string in hand, rotate the stone in a horizontal circle. During rotation, if the string breaks at a certain stage, the stone is found to fly off tangentially.
 
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