In an elastic collision, both momentum and kinetic energy are conserved. However, in an inelastic collision (where the coefficient of restitution is smaller than 1) only momentum is conserved.
This Demonstration models inelastic collisions between two hard spheres of equal density. You can vary the coefficient of restitution (), the relative mass (), and the impact parameter , the distance of closest approach of the centers if the spheres are undeflected. For a head-on collision, . You can observe the collision in slow motion in either the laboratory or the center-of-mass frame of reference. The spheres leave dotted contrails to help you trace their trajectories.