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

0 votes
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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?
asked Oct 27, 2013 in Intro College Physics by anonymous

1 Answer

0 votes

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 PhysicsDude (21,750 points)
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