Suppose we have a universe consisting of three planets. Their velocities and positions at t=0, and the gravitational constant are known. One can calculate, using differential equations, the positions of the planets at any time. Assuming that the only force acting on the planets is gravity and that whenever the planets collide it is in an completely elastic manner, will there be conservation of mechanical energy? Also, if one wants to calculate the potential energy at any time t, how can that be done? What do we take as "reference point"? The center of mass of all masses in the universe ?