Available Potential Energy

The blue is more dense water and the white is less dense. If we release the water from the configuration in the first panel, with the center of gravity indicated by the black dot, we see that the difference in density leads to the gravity wave motion in the second and third panels. The motion in the second and third panels is caused by the conversion of potential energy into kinetic energy. The final panel shows how the center of mass has been lowered, and we have reached a minimum configuration of potential energy.