Center of mass separate bodies, such as the planets of the Solar System, the center of mass may not correspond to the position of any individual member of the system. The center of mass is a useful reference point for calculations in mechanics that involve masses distributed in space, such as the linear and angular momentum of planetary bodies and rigid body dynamics. In orbital mechanics, the equations of motion of planets are formulated as point masses located at the centers of mass. The center of mass frame is an inertial frame in which the center of mass of a system is at
Mass–energy equivalence equivalence of mass and energy". In addition, Einstein used the formula , with being the energy of a system of mass points, to describe the energy and mass increase of that system when the velocity of the differently moving mass points is increased. In June 1907, Max Planck rewrote Einstein's mass–energy relationship as , where is the pressure and the volume to express the relation between mass, its "latent energy", and thermodynamic energy within the body. Subsequently, in October 1907, this was rewritten as and given a quantum interpretation by Johannes Stark, who assumed its validity and correctness ("Gültigkeit"). In