Re: Newton's Gravity

In article <<lu6p15F9qp8U1@mid.dfncis.de> I wrote

if we have a pair of masses M1 and M2,
fixed in position (with respect to a Newtonian inertial reference frame
(IRF), to keep things simple) some distance apart, with M1 not equal to
M2, is there any good reason to think that the gravitational force of M1
acting on M2 is equal in magnitude and opposite in direction to the
gravitational force of M2 acting on M1?
 
There are actually a couple of useful lines of reasoning, each of which
suggests that the answer is "yes":
 
[[...]]
 
    So, to summarize, we've shown that if the force of M1 on M2 were
    NOT equal-in-magnitude-and-opposite-in-direction to the force of
    M2 on M1, then you could violate the laws of conservation of momentum
    and conservation of momentum, and build a perpetual motion machne.

I'm sorry, I garbled that last quoted sentence.  What I meant to write
was this:

    So, to summarize, we've shown that if the force of M1 on M2 were
    NOT equal-in-magnitude-and-opposite-in-direction to the force of
    M2 on M1, then you could violate the laws of conservation of momentum
    and conservation of energy, and build a perpetual motion machne.

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