Write an expression for the potential energy of a body of mass m in the gravitational field of the earth and moon. let me be the earth's mass, mm the moon's mass (where 81 m m e m = ), r the distance from the earth's center, and r the physics 113p, summer 2015 study guide module 9 © 2015 university of rochester 4 all rights reserved distance from the moon's center. the distance between earth and moon is about 384,400 km.