In physics, the center of mass or barycenter is the weighted average location of all the mass in a body or group of bodies. Various important calculations in mechanics become simplified when quantities are referenced to the center of mass, or when the entire mass of a body is treated as if it is concentrated at the center of mass.
In the case of a rigid body, the center of mass is fixed in relation to the body, and it does not necessarily coincide with the geometric center. Nor does the center of mass necessarily coincide with any point on the body, as is often the case for hollow or open-shaped objects, like a horseshoe. In the case of a loose distribution of particles or bodies, such as the planets of the Solar System, the center of mass of the entire group may not correspond to the position of any individual member.
The mass center often obeys simple equations of motion, and it is a convenient reference point for many other calculations in mechanics, such as angular momentum and moment of inertia. In many applications, such as orbital mechanics, objects can be replaced by point masses located at their mass centers for the purposes of several types of analysis. The center of mass frame is an inertial frame in which the center of mass of a system is at rest at the origin of the coordinate system. Wikipedia, Barycenter