Center-of-momentum Frame

In physics, a center-of-momentum frame (zero-momentum frame, or COM frame) of a system is any inertial frame in which the center of mass of the system is at rest (has zero velocity and hence momentum). Note that the center of momentum of a system is not a location, but rather defines a particular inertial frame (a velocity and a direction). Thus "center of momentum" already means "center-of-momentum frame" and is a short form of this phrase.

A special case of the center-of-momentum frame is the center-of-mass frame: an inertial frame in which the center of mass (which is a physical point) is at the origin at all times. In all COM frames, the center of mass is at rest, but it may not necessarily be at rest at the origin of the coordinate system.