Frictional force opposes motion of body. If force equal to frictional force is applied to body in direction of motion resultant (net) force on body will become zero. Hence, body will continue motion with constant velocity.

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When body is at rest, resultant force acting on it is zero.

Thus, when body is at rest or moves with constant velocity both situation are equal because in both case resultant force acting on it is zero.

When resultant force acting on body is zero, body at rest will remain at rest and body in motion will continue its motion with constant velocity. This property of body is called inertia. Thus, property of a body to oppose change is called inertia. Mass of body is measure of inertia. Heavier body has more inertia and lighter body has less inertia.

A body cannot change its condition at rest or in motion at its own.