Inertia is the ability of the body to remain in the state or in uniform motion unless some external force acts on the body.
The inertia of the body is direly proportional to the mass of the body.
Thus, the greater the mass of the body greater is its inertia.
Three types of inertia:
In the state of rest, the inertia of the body resists the cause of motion. This inertia is known as resting inertia.
In the state of uniform motion, the inertia resists the cause of changing of motion. This inertia is known as motion inertia.
Some external force is applied to the body to change the direction of the motion of the body, the inertia that opposes this change in direction is known as directional inertia.