# Center of mass

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In physics, the center of mass or barycenter of a body is a point in space where, for the purpose of various calculations, the entire mass of a body is concentrated. In common usage, the center of mass is also called the center of gravity, because the weight of a body acts as if it were concentrated there in a uniform gravitational field. In a non-uniform field, the center of mass no longer serves as an exact center of gravity, so physicists often distinguish the center of gravity as a separate concept.

The center of mass of a body may be defined as the average location of the mass distribution. In the case of a rigid body, the center of mass is fixed in relation to the body, and it may or may not coincide with the geometric center. In the case of a loose distribution of masses, such as the planets of the Solar System, the center of mass may not correspond to the position of any individual mass.

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 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.

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