The inertia of a body is defined as that property of it, which tends to resist a change in its state of rest or of motion. More massive bodies have greater inertia. Hence mass is defined as a quantitative measure of inertia.
Property of inertia may be of two kinds (i) inertia of rest and (ii) inertia of motion.
(i) Inertia of rest: The inertia of rest is the property of material bodies to persist in their state of rest. It is a common experience that a body remains at rest unless something is done to change the state. The body suddenly does not move out of its own accord.
Therefore; force is the external agency, which has to be applied on a body in order to change its state of rest or of motion.
Example : When a motor car or any vehicle starts suddenly and rapidly gains speed, a passenger standing or sitting unmindfully leans backwards. The lower part of his body being in contact with vehicle, shares its motion, but the upper part due to inertia of rest, tends to stay at rest. The man thus leans backwards.
(ii) Inertia of motion: The property of a body to continue in its uniform motion along a straight line is called its inertia of motion.
Example : When a moving motor car or any vehicle suddenly stops, a passenger standing or sitting unmindfully falls forward. His lower part being in contact with the vehicle stops with it, but the upper part of the body continues moving forward so that the person leans forward.