# Is the net force of an isolated system zero?

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.

• I don't understand how an internal perturbation can occur. I am only considering cases where there is absolutely no influence from the outside. Nov 18, 2021 at 10:45
• @franz. It would be very helpful if you could help me as I am not the expert in this. Nov 18, 2021 at 10:50
• 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/… Nov 18, 2021 at 10:55
• Thanks. That is exactly what I needed. Sorry for writing my question so badly. Nov 18, 2021 at 10:57

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.