Explain the difference between elastic and inelastic collisions.

In all collisions, provided there are no external forces, momentum is conserved. This is the law of conservation of momentum. However, kinetic energy is not conserved in all collisions. An inelastic collision is one in which not all of the kinetic energy is conserved. An example could be a collision between two cars, whereby both had kinetic energy before colliding, and none afterwards as they came to rest. A perfectly elastic collision is one in which all kinetic energy is conserved. An example of this is interactions between molecules, or in a Newton's cradle.