# Stages of collision

There are three distinct identifiable stages in collision, namely, before, during, and after. In the before and after stage, the interaction forces are zero. Between these two stages, the interaction forces are very large and often the dominating forces govern the motion of bodies. The magnitude of the interacting force is often unknown; therefore, Newtonâ€™s second law cannot be used; the law of conservation of momentum is useful in relating the initial and final velocities.