Question

g=GM/r2 how to prove this?

Answer 1

Answer:

This looks like an order rather than a question. But anyway…

Let a point object with mass m be at a distance r from a point mass M .

Then by Newton’s second law, mg=GmM/r2 , so g=GM/r2 .

Note that you can use the centre of mass for an extended symmetrical mass, but that requires proof. So this would roughly apply to small bodies on the surface of the Earth. A proof of that is not so trivial.