Here, by system I mean all the objects in consideration. Net force means the vector sum of all forces present in the system. For example, let's say there are two magnets somewhere in a vacuum space where no force from outside is present. This two magnets are the system. There will be some force on magnet-1 and some on magnet-2. Is the vector sum of all the forces zero? Or can it be non-zero.

I think it must always be zero but a certain FEM simulation is giving non-zero result. I could not find any discussion about it. I am hoping for some helpful opinion here.

  • $\begingroup$ I don't understand how an internal perturbation can occur. I am only considering cases where there is absolutely no influence from the outside. $\endgroup$ Nov 18, 2021 at 10:45
  • $\begingroup$ @franz. It would be very helpful if you could help me as I am not the expert in this. $\endgroup$ Nov 18, 2021 at 10:50
  • $\begingroup$ rethinking what I just wrote, I think the internal forces in a closed system should cancel due to newtons third law. check this article: physics.stackexchange.com/questions/336028/… $\endgroup$
    – franz
    Nov 18, 2021 at 10:55
  • 1
    $\begingroup$ Thanks. That is exactly what I needed. Sorry for writing my question so badly. $\endgroup$ Nov 18, 2021 at 10:57

2 Answers 2


From Newtons third law "for every action there is an equal and opposite reaction", the net result of forces between any two of the objects is zero.

That is, if object A pulls object B with $10$N then object B pulls object A with $10$N in the opposite direction. Therefore the sum of all the forces (or all the pairs of forces) is zero.



by Newton's 3 law of motion, an isolated system is a system where no external forces act on both of the objects simultaneously. So, net force=0 as any force between them will have some action-reaction pair


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