The official story tells us that is a “definition” and that its conservation is a theorem that follows from Newton’s third law. All true, but there is another way of reading the story, today preferred by physicists and educators: the principle of conservation of momentum is more fundamental than Newton’s laws themselves.
Indeed, it holds even where Newton’s laws do not — in special relativity, in quantum mechanics — and it can be derived, via Noether’s theorem, from the fact that the laws of physics are invariant under spatial translations. There is no privileged point in space, and from this symmetry alone follows, as if by magic, the conservation of .
It is one of the rare cases in which an almost metaphysical principle (“space is homogeneous”) translates into a quantitative law of enormous predictive value. Conserving momentum, at bottom, means accepting that space has no centre (Feynman, Leighton and Sands 1963; Battimelli and Stilli 1999).
For the full list of works cited: Riferimenti bibliografici.
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Topics: Quantità di moto e urti Concepts: Conservazione della quantità di moto · Terza legge di Newton
Related exercises: Razzo nel vuoto · Terzo principio rotto · Terza legge a senso unico