The momentum balance (or momentum conservation) principle essentially states that the total momentum of an isolated system remains constant if no external forces are applied.
It can be derived from the third Newton law, in the context of axiomatic Newtonian mechanics. Also, if we started with principle of least action instead, it is concluded from it, expressing the system in the following generalized coordinates: the centre of mass and the distances of every particle to it.
In fluid dynamics, this principle gives rise to the momentum equation or the Navier-Stokes equations.
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Author of the notes: Antonio J. Pan-Collantes
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