Asked by sdf

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

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Answered by GPT-5 mini AI
A necessary condition for conservation of (linear) momentum of a system is that the net external force acting on the system is zero (or that no net external impulse acts on it during the time interval). In symbols:

dP/dt = F_ext = 0 ⇒ P = constant.

Equivalent statements: the system must be isolated from external forces (or closed with only internal forces that satisfy Newton’s third law). For conservation of momentum in a particular direction, the net external force in that direction must be zero.

(If mass is changing, the simple statement needs modification — one must account for momentum carried in or out by mass flow.)