Why state of body and body at rest and body moving with constant velocity are equal ? What is inertia ?

Explanation

Frictional force opposes motion of body . If force equal to frictional force is applied to body in direction of motion resultant force on body will become zero. Hence body will continue motion with constant velcoity
Thus when body is at rest or moves with because in both case resultant force acting on it is zero.
Thus property of a body to oppose change is called inertia . mass of body is measure of inertia
A body cannot change its condition at rest or in motion at its own.