Question

Suppose the Moon is at a (center-to-center) distance D from Earth. Find an exact expression for the magnitude of the tidal force applied by Earth to our rock of mass m in terms of D, m, the mass of Earth Me (input as M_e), and the lunar radius Rm (input as R_m).
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Also
Assuming that D is much greater than Rm find an approximate expression for the same tidal force, using Newton's approximation method