If u1 and u2 are the velocities of two moving bodies in the same direction before impact and v1 and v2 are their velocities after impact, then coefficient of restitution is given by
(v1 - v2)/(u1 - u2)
(v₂ - v₁)/(u1 - u2)
(u1 - u2)/(v1 - v2)
(u₂ + u₁)/(v₂ + v₁)
