Question

Darwin proposed that the Moon formed far closer to Earth than it is now. Let’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).

Answer 1

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.