restitution, coefficient of In one dimensional elastic collision between two spheres, the relative velocity before the collision is equal to relative velocity after the collision. In an inelastic collision , the relative velocity after the collision is less than the relative velocity before the collision.

relative velocity before collision = - e ´ relative velocity after collision

The constant e measures the inelasticity of collision and is called the coefficient of restitution. It depends on the material of the spheres.