The elasticity of the collision is related to the ratio of the relative velocities of the two colliding objects after and before the collision:

k = (v_2f - v_1f) / (v_1i - v_2i)

The elasticity is related to the type of collision as follows:

Credits:

Modified from Andrew Duffy's web page at Boston University. The animation uses the Physlet Java Applets of Wolfgang Christian and Mario Belloni.