Posted by sam on .
Darwin proposed that the Moon formed far closer to Earth than it is now. Letâ€™s see just how close it could possibly be to Earth and survive. In particular, in the next 4 problems you will find the distance from Earth at which the tidal force due to Earth is sufficient to lift a rock off the lunar surface. To do this, imagine a rock of mass m on the lunar surface. Write an expression for the gravitational force applied by the Moon to the rock, in terms of the lunar mass Mm (input as M_m) and the lunar radius Rm (input as R_m).

Astronomy 
drwls,
Let the moon rock mass be m
F = G*m*(M_m)/(R_m)^2
is the force that hold the rock to the moon. It is the same as its weight.
Learn the universal law of gravity.