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We know how (and why) conservation of momentum works in single-particle and two-particle systems. What happens when a system contains

It's very simple, actually. Newton's
Laws (particularly the Third
Law) guarantee that every interaction involves *exactly two*
particles (A pushes B, B pushes A) and we have already established
that momentum will always be conserved in every two-particle
interaction. That's all there is to it! Even if there are uncountable
billions of particles in a system, every interaction can be broken
down into two-particle interactions, and momentum is conserved in
every one of them. Therefore, momentum must be conserved in *every
isolated system* - no matter what it contains.

last update December 12, 2005 by JL Stanbrough