# Is momentum always conserved? Even if there is net ext. force acting on system?

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I understand that there are two situations one about an isolated system in which there is no ext. force and so deltap=0

and one non-isolated system in which there is an external force acting so deltap=Impulse

My question is, is momentum always conserved in both these situations?

It depends on how you define your system. If you include the external object which exerted the external force in a grander system, then the momentum of that system would be conserved. -- that is if you define the whole universe as your system, then all the momentum of all matters inside the universe would sum to a constant and be conserved.

When you have $\Delta P=Impulse$, the  "reaction" force (Newton's 3rd Law pair) which has same magnitude and opposite direction would act on the external object creating a negative impulse on that object. Therefore, if you include that object into the system, the total P would be conserved.

answered Oct 27, 2013 by (21,750 points)