Co-efficient of Restitution

The co-efficient of restitution, e, is the fraction of energy preserved after a collision between two objects. Drop a superball onto concrete and it bounces back almost as high as it was dropped from; drop a lump of clay from the same height and it hardly bounces at all.

e ranges from 0 (a perfectly inelastic collision) to 1 (a perfectly elastic collision):

e = -vf / vi, where vi is initial velocity and vf is final velocity — e = -v_f / v_i, where v_i is the initial (approach) velocity and v_f the final (rebound) velocity.

For bulk materials, e is typically in the range 0.1–0.3, though this depends heavily on the material and the chute face properties. Some reference values:

Stainless steel balls: e ≈ 0.95
Baseball: e ≈ 0.55
Golf ball: e ≈ 0.85
Lump of clay: e ≈ 0.0

See References for further reading.

Between particles. The restitution described here governs bounce off chute faces and pulleys. Bounce between particles is controlled separately, and is off unless you switch it on – see Particle – Particle Damping (Restitution).