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Q & A: Newton's Third Law and Magnetism

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Most recent answer: 08/07/2013
Q:
I have always been a little curious about the equal and opposite reaction to magnetism. I understand (to a degree) that there is a pull when poles are facing one direction and a push when facing the opposite direction. I also understand that these are equal opposite reactions. What I don't understand, is that it seems if they are incapable of occurring simultaneously. It seems to me that if this is true, then Magnetism is the one thing (that I can think of) that does not coincide with Newton's equal and opposite reaction theory. Obviously I am missing a whole lot here and I am sure there is a perfectly logical explanation, so I am curious as to what it might be. Thank you so much!
- Adam (age 38)
Sandwich, MA
A:

Yes, the problem is not with your understanding of magnetism but of Newton's Third Law. The usual language about "action" and "reaction" is just confusing. The law is very simple. The force between two objects is the same size on each and points opposite directions on each. In other words, the sum of the forces is zero. Take two magnets and they either repel each other (opposite direction forces) or attract each other (opposite direction forces) depending on their orientation. In either case, the Third Law is obeyed. That's all there is to it. That both those possibilities exist has nothing to do with the Third Law. It applies equally to gravity, for which the only forces are attractive.

A more general picture of Newton's Third Law is to just say that total momentum is conserved. This is important for cases where boh electrical and magnetic fields are present. In those cases, adding up the forces on the ordinary "objects" doesn't give zero, because there's a bit of changing momentum in the fields themselves.

Mike W.


(published on 08/07/2013)

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