angle of incidence angle between the direction of the incident relative velocity of the contact points and the common normal direction. Direct or normal collisions have zero angle of incidence, whereas oblique collisions have a nonzero angle of incidence.

angle of rebound angle between the direction of the relative velocity of the contact points at separation and the common normal direction.

attractor steady state solution that is approached asymptotically with increasing time if the system has small dissipation.

coefficient of friction upper limit on ratio of tangential to normal force at contact.

coefficient of stick geometric parameter specifying ratio of tangential to normal force for stick.

collinear (or central) impact configuration colliding bodies oriented so that each center of mass is on common normal line passing through the point of initial contact.

common normal direction normal to common tangent plane that passes through contact point C.

common tangent plane If at least one of the bodies has a topologically smooth surface at the contact point, this is the plane that is tangent to the surface at the point of initial contact. Usually both bodies have smooth surfaces around their respective points of contact, so they have a common tangent plane.