In an elastic collision, both momentum and kinetic energy are conserved. This Demonstration models collisions between two hard spheres of equal density. You can vary 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.