Finally, consider the gravitational force generated by a spherically symmetrical massive object. The magnitude and direction of such a force are given by Newton's law of gravity FG = - G m₁m₂/r² x r where ds = dr r; G, m₁, and m₂ are constants; and r > 0. Find rf U(rf) – U(ro) =-∫ FG. ds ro Express your answer in terms of G, M₁, M₂, ro, and rf.