As defined in Chapter 7, gravitational potential energy is U = mgy and is positive for a body of mass m above the earth’s surface (which is at y = 0). But in this chapter, gravitational potential energy U = – Gm_Em/r, is which is negative for a body of mass m above the earth’s surface (which is at r = R_E). How can you reconcile these seemingly incompatible descriptions of gravitational potential energy?