18.
REASONING
The magnitude of the gravitational force that each part exerts on
the other is given by Newton’s law of gravitation as
2
12
/
FG
m
mr
=
.
To use this
expression, we need the masses
m
1
and
m
2
of the parts, whereas the problem statement
gives the weights
W
1
and
W
2
.
However, the weight is related to the mass by
W
=
mg
, so
that for each part we know that
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 Spring '08
 SOFFER
 Mass, General Relativity, Newton's law of universal gravitation, 12 m

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