1. The problem statement, all variables and given/known data Show that the variation of gravity with height can be accounted for approximately by the following potential function: V(z)=mgz(1-z/R) Where R is the radius of the earth and z the height above the surface. 2. Relevant equations r=R+z V=-GmM/r F=GmM/r^2 3. The attempt at a solution First, I said define z such that r = R + z. We have the potential energy function for the earth as V(r)=-GmM/r, so V(z)=-GmM/(R+z). I then expanded this around z=0 and took the first two terms in the series to get: V=-GmM/R + GmMz/R^2 and you can factor this to get V=GmM/R * [1-z/R] but the force F given by the potential energy function is GmM/R^2 at the surface, so this is equal to mg, so g = GM/R^2 So the V function is then V = mgR[1-z/R] And here I am stuck. It appears that I am still taking the centre of the planet as my reference point. Can someone help me? I'm stuck. Somehow I need to redefine the reference point so that V = 0 for z = 0. How can I do this?