Quote:
Originally Posted by michaellxv
Gravity between 2 bodies acts as if the masses were points of mass at the centre of gravity. Therefore I don't see how gravity is going to cause either body to necessarily start rotating.
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It doesn't. The gravitational force between Earth and Moon isn't what makes either of them spin around their respective axes.
EDIT I should add, if you abandon the point mass assumption (because it is inaccurate and only an approximation) you will have to acknowledge the effects of tidal deformation, which results in any kind of rotation the Moon might have had initially to eventually become synchronous. If the mechanics of the Moon's genesis had not given it any kind of rotation (i.e. a pure translational path around Earth) those tidal forces would have induced a spin resulting in synchronous rotation. Hence, since Earth and Moon are not point masses you can say that gravitation (via tidal forces) can indeed induce rotation (at least in the smaller one of the two).
Cheers
Steffen.