1. Definitions
A collision is an interaction between two or more objects that occur in limited time and space. During a collision, the forces between the objects may be complicated. We examine just what goes into a collision and what comes out of the collision. We assume that the net external forces to the system of the interacting objects are negligible. Therefore, they will produce negiligible impulses, and thus the momentum of the system is conseved. That is The total momentum of the system before the collision is equal to the momentum of the system after the collision.