The coefficent of restitution cor is a measure of the bounciness of a collision between two objects;how much of the kinetic energy remains for the objects to rebound from one another much energy is lost as heat ,or work done deforming the objects
the coeffcient ,e is defined as the ratio of relative speeds after and before the impact,taken along the line of the impact

coeffcient of restitution(e)= relative speed after collisio0n/relative speed before collision