1. The problem statement, all variables and given/known data "How much energy is released when a sphere of constant density (p) with mass (M) and radius (R) is put together gravitationally? What you should do is to think of the energy released when a shell is brought in from infinite distance (where potential energy of zero) to the current surface of radius r of the sphere. What is the gravitational force on a sphere as it moves inwards, what is the differential mass of the infinitesimally thin shell, what distance is it brought to, and hence what is the differential energy for bringing in the shell? Then, integrate the differential energy over radius to get total energy, 2. Relevant equations 3. The attempt at a solution I am pretty lost on the solution. I have: F_g=GMm/r^2 Integrated from infinity to r (GMm/r^2) This, however, gives potential, energy, which is 0. Any help would be appreciated, i'm very lost on where to even begin on the problem. Thanks!