What is "coefficient of restitution"

The coefficient of restitution (COR) is a measure of the "restitution" of a collision between two objects: how much of the kinetic energy remains for the objects to rebound from one another vs. how much is lost as heat, or work done deforming the objects.

The coefficient, e is defined as the ratio of relative speeds after and before an impact, taken along the line of the impact:

$\text{Coefficient of restitution } (e) = \frac{\text{Relative velocity after collision}}{\text{ Relative velocity before collision}}$

Alternatively, this may be expressed as: Relative velocity of separation = e × Relative velocity of approach

The mathematics were developed by Sir Isaac Newton in 1687. It is also known as Newton's experimental law, though it is really just another way to think about conservation of energy and momentum.