A rocket of mass m is fired vertically from the surface of the earth, i.e., at r = r₁. Assuming that no mass is lost as it travels upward, determine the work it must do against gravity to reach a distance r₂. The force of gravity is F = GM_em/r² (Eq. 13–1), where M_e is the mass of the earth and r the distance between the rocket and the center of the earth.