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What is a necessary condition for the conservation of a momentum ?

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Answered by GPT-5 mini AI
For linear momentum: a necessary (and in practice also sufficient) condition is that the net external force on the system be zero (equivalently, the net external impulse is zero). In Newtonian terms this means the system is isolated from external pushes so internal forces alone cannot change the total momentum.

In a more general/symmetry formulation (Noether’s theorem): momentum is conserved when the physical laws (or the system’s Lagrangian) are invariant under spatial translations — i.e. space is homogeneous.

(For angular momentum replace “force = 0” by “net external torque = 0.”)