Coefficient of Restitution
A fractional measure of elasticity in a collision, defined as ratio of relative separation speed to relative approach speed.
Governing formula
e = (v2_f - v1_f) / (v1_i - v2_i)
SI unit
Dimensionless (0 to 1)
In depth
For perfectly elastic collisions e = 1; for completely inelastic collisions e = 0.
Examples in the real world
A tennis ball bouncing back up to 80% of its drop height has e = √(0.80) ≈ 0.89.