Newton’s Law of Gravity
At the earth’s surface, we have
Fgravity = G*Mearth*m / Rearth2
where the distance between an object on the earth’s surface and the earth (center to center distance) is the radius of the earth.
Note that G, Mearth and Rearth are all constant, so that near the earth this reduces to
Fgravity = mg where g = G*Mearth/Rearth2 .