Coefficient of Restitution (cor) of colliding bodies describes how elastic or inelastic a collsion is. But, most internet sources (the golf-related sites) and http://en.wikipedia.org/wiki/Coefficient_of_restitution", describes cor as a property of a particular object. Isn't this wrong? The cor depends on the velocities of both the colliding bodies. So, the cor should be described as a property of a collision. For example, if I drop a golf ball on fairway and then, in sand; obviously, the ball will bounce more on the fairway. Here, it has 2 different cors (one close to elastic & other close to inelastic). In one-dimensional collsion, the cor is given by cor = ( v2f - v1f )/( v1i - v2i ) Here, we use positve and negative signs to deal with directions of velocity vectors. What about two-dimensional collisions?