Symmetry and conservation
… all conserved quantities come from symmetries in our universe. Because the laws of physics are the same no matter what direction you look in, angular momentum is conserved. Because they’re the same whether you move left or right, regular momentum is conserved. Because they’re the same now as they will be later in time, energy is conserved. Energy and momentum conservation come from two separate symmetries; …
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— Physics Stack Exchange
— Colin Fredericks
That is, conservation of momentum is a consequence of the fact that the laws of physics do not depend on position; this is a special case of Noether’s theorem.
— Wikipedia on Momentum
2012.11.17 Saturday ACHK