To simulate impact forces, you must specify a value for the coefficient of restitution. The coefficient of restitution is the ratio of the velocity of two entities after and before a collision. Typical coefficients of restitution can be found in engineering textbooks, or from empirical studies.

Coefficients of restitution depend on factors including material properties, body geometry, and impact velocity. Applying a coefficient of restitution to your mechanism simulates nonrigid properties in a rigid body calculation.

For example, a perfectly elastic collision has a coefficient of restitution of 1. A perfectly inelastic collision has a coefficient of restitution of 0. A rubber ball has a relatively high coefficient of restitution. A wet lump of clay has a value close to 0.