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Mass is constant in an isolated system, but what about velocity or momentum? Because momentum = mass x velocity, should they remain unchanged like mass in an isolated system.

Please help me get this question off my head. Thank you so much!

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Many quantities that can be ascribed to the whole system (mass, momentum, electrical charge, angular momentum) are, because they are conserved, always unchanged in an isolated system. Velocity does relate to momentum, and classically the velocity of center-of-mass would be conserved (but there are relativistic corrections).

The isolation, though, is a problem. No part of this universe is isolated from, for instance, gravitation. Any object on Earth is in constant acceleration as the world turns, so an otherwise-isolated system that changes its elevation is subject to a pseudoforce called 'coriolis'.

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In general, the momentum of a system of particles is a conserved quantity, as is the energy. The velocity of a system of particles may be taken as the velocity of the center of mass of the system, and it is, in general a conserved quantity since it is a scalar multiple of the momentum of the center of mass of the body, which is conserved in the absence of external forces.

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