1. The problem statement, all variables and given/known data Ok, I am deriving the change in gravitational potential energy at the surface of the earth and have met with some difficulties. 2. Relevant equations [tex]g=GM/r^2[/tex] [tex] P.E.=-GMm/r[/tex] 3. The attempt at a solution [tex] \delta P.E. = -GMm/r_f+GMm/r_i[/tex] Where [tex] r_f > r_i[/tex] Since gravitational field strength is approaximately a constant for small changes of r, [tex] \delta P.E. = -mgr_f + mgr_i[/tex] But this gives me a negative answer! How can my change in gravitational potential energy be negative when I bring my object from a lower position to a higher poistion in the gravitational field. I have probed the mathematics of the above proof, but it all makes sense to me since g is a constant.