Inertia.

It is the inherent property of all the bodies by virtue of which they cannot change their state of rest or uniform motion along a straight line on their own.

Inertia is not a physical quantity, it is only a property of the body which depends on mass of the body.

Mass gives an idea of measure of inertia.

Types of Inertia

1. Inertia of rest : It is the inability of a body to change by itself, its state of rest. This means a body at rest remains at rest and cannot start moving by its own.

Example: When a bus starts suddenly, the passenger tends to fall backward on account of inertia of rest of upper part of the body as explained above.

2. Inertia of motion : It is the inability of a body to change itself its state of uniform motion i.e., a body in uniform motion can neither accelerate nor retard by its own.

Example : (i) When a bus  stops suddenly, a passenger sitting inside tends to fall forward. This is because the lower part of his body comes to rest with the bus  but the upper part tends to continue its motion due to inertia of motion.

3.Inertia of direction : It is the inability of a body to change by itself direction of motion.

Example :The rotating wheel of any vehicle throw out mud, if any, tangentially, due to directional inertia.