An Etymological Dictionary of Astronomy and Astrophysics
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A fundamental law of physics which states that the momentum of a → physical system does not change in the course of time if there are no external forces acting on the system. It is embodied in → Newton's first law. This principle shows that the interaction of bodies composing a → closed system leads only to an exchange in momentum between the bodies but does not affect the motion of the system as a whole. More specifically, interactions between the composing bodies do not change the velocity of the system's → center of mass.

→ conservation; → momentum.