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A water jet impinging upon a fixed pelton wheel does not in fact
cause some contra-rotation of the Earth mass - no indeed. The force
is reacted much, MUCH closer to home.
In his inimitable style, Brian writes:
Newton kindles the juvenile imagination when he relates the tenacious
symphony of the spheres. But when this same mass in consideration
is quasi-infinite in the context, that looks more like an article of
faith provided by an educator: the tender wheels of the student's
traction falter at the concept, do they not? Better to ask the
young to imagine what they can imagine.
A water jet impinging upon a fixed pelton wheel does not in fact
cause some contra-rotation of the Earth mass - no indeed. The force
is reacted much, MUCH closer to home. So teachers who talk of the
Earth moving, are in the position of discussing Angels Dancing Upon
Pin Heads - and I disapprove the dishonesty of it.
I take your point that the reaction of Earth is unmeasurable. But to
prefer that we tell our impressionable young charges that Newton's
third law applies only when neither mass is too large is, in my
opinion, to greatly sully the concept of universal physical law and
to beg the question, "How large is too large?" I think if I had been
told such things as a student, I would have found physics revoltingly
arbitrary and might have expressed my hunger for practical
applications of mathematics in engineering instead.
--
John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm