A rocket of mass $m$ is fired vertically from the surface of the earth, i.e., at $r=r_{1} .$ Assuming that no mass is lost as it travels upward, determine the work it must do against gravity to reach a distance $r_{2} .$ The force of gravity is $F=G M_{e} m / r^{2}$ (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.