This question is really starting to bug me!! Consider for example Io is orbiting Jupiter and the force of the gravity pulling the moon towards the planet vs the momentum pulling in the other direction creates friction in the moon's core. First of all, this should heat up the moon. But where exactly did the energy come from? Gravity isn't an example of energy, energy can only be 'held' in gravity as a potential energy if a force acts against it.The force against it in this case is the 'centrifugal' force acting on the moon. You'd assume that it is caused by the kinetic energy of the moon, only it seems as if it never runs out even though it obviously heating it? Where does the energy actually come from? It seemed like a really trivial and simple question at first but the more I thought of it the more it confused me. I have a limited understanding of physics as I have just turned 14 and it's early in the morning. :tongue2: P.S about 7:53 where I live on a weekend not 3:53 soo yeah.