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When I state the third law in class, I begin: "when an object A exerts a
force on object B..."
This formulation evades the issue of whether the third law applies to the
sum of the forces. And I am glad to evade that question: I care about the
sum of the forces that act ON object A because that sum determines object
A's acceleration. I don't care about the sum of the forces exerted BY
object A because it is likely that they are acting on an assortment of
objects, each which are experiencing their own collection of forces.
I have often told my students that I am unaware of any exceptions to this
law and that if there were any exceptions, they would violate the law of
conservation of momentum. But now I see from Bruce's post about the
electron adn proton pair that I have some reading to do.