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"I'm not sure I'm comfortable with the electron/proton example as a
violation of the third law." I of course agree totally with the fact that
the right way (and the only feasible way) to calculate magnetic forces is
to first calculate the field made by other (moving) charges and then
calculate the cross-product force. But the fact remains that you end up
with two forces that are not F and -F, which calls for some discussion.
How do you feel about the Coulomb force law? Surely there too one should
first calculate the field made by other charges and then calculate the
force (for one thing, retardation effects require this approach).
Yet in
the intro course one rarely requires this of students, and it's probably
common to say "See? Here's an example of Newton's 3rd law."
But then what
do you say when the two magnetic forces don't match?