Natural tendency to resist change in the state of motion or rest or direction is called inertia. Changes of state means from rest to motion or from motion to rest or from moving in one direction to other. Inertia of a body is proportional to mass of that body. Due to inertia of rest a body at rest has a tendency to remain at rest. Some external force is required to change its state from rest to motion. Force required is more if the inertia of the body is high (i.e. heavy mass). A moving body has inertia of
motion & inertia of direction. Thus its tendency is to oppose a change in state from motion to rest or change in one direction to other direction.