# 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?
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.