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I have stated that Newton's mechanics and his theory of gravitation are
but approximations to reality and whose limitations are now
known ^{}. So it
might be questionable to use *F* = *ma* and
*F*_{grav}* = m M G/r*^{2}
as basis to any argument as was done above. Einstein was careful to use
these expressions only in situations where they are
extremely accurate (small speeds compared to *c* and small gravitational
forces). In these cases the inertial and gravitational masses are
identical, as shown by experiment. Then he postulated that the
same would be true under *all* circumstances. This statement, while
consistent with Newton's equations, cannot, in a strict logical sense,
be derived from them.

*Jose Wudka*

*9/24/1998*