When is momentum conserved?

Answer 1

its not possible.. momentum is always conserved

You could say that momentum, in its classical definition, is not conserved at relativistic velocities. Momentum is conserved at relativistic speeds if momentum is redefined as; p = γmov where mo is the "rest (invariant) mass" and γ is the Lorentz factor, which is equal to γ = 1/√(1-ʋ2/c2) and ʋ is the relative velocity. Some argue that the relativistic mass, m' = γmo, is unnecessary, in which case the proper velocity,

defined as the rate of change of object position in the observer frame with respect to time elapsed on the object clocks (its proper time) can be used.

Proper velocity is equal to v = γʋ, so p = mov. mo here is the invariant mass, where before it represented the "rest mass."

The problem with Newton's p = mv, is that with this definition, the total momentum does not remain constant in all isolated systems, specifically, when dealing with relativistic velocities. Mass and or velocity is dependent on the relative velocity of the observer with respect to the isolated system.

It is important to add that with this new definition momentum is conserved. With that said, my point is not to argue that momentum is not always conserved but to simply offer an explanation for the relatively (no pun intended) common statement "momentum is not conserved in ALL isolated systems" which could be where the original question stems from.

Answer 2

Well, momentum is always conserved in a closed system.

Answer 3

during elastic collision