Conservation of Momentum in Collisions

You have now seen that collisions are any interactions between two objects.  In all interactions in a closed system, momentum is conserved.    Some collisions are elastic, in which the objects bounce off each other in such a way that kinetic energy is also conserved.  When objects bounce but kinetic energy is not conserved, with some being converted to heat, sound or other mechanical vibrations, these are considered inelastic collisions. When the objects actually stick together or move together after the collision, these are called perfectly inelastic collisions.  To solve all problems involving collisions, your first step is to set the sum of the momentum of both objects before the collision equal to the sum of the momentum of both objects after the collision.  Remember that momentum is a vector, so the direction is very important.

For more information on collisions view the video clip Inelastic and Elastic Collisions from Discovery Education™ streaming.