In explaining why it does not make sense for two objects feeling each other's gravity to simply stay in place, the book "Simple Nature" (Ben Crowell, April 2010 edition) states that: My question is, why does one planet moving and not the other violate the conservation of energy? I could say that some of the initial gravitational energy between the planets is converted into kinetic energy for only one of the planets. I believe that it can be explained in terms of Newton's action-reaction law, but at this point in the book, forces have not even been discussed yet. What am I missing here? Any light on the matter would be much appreciated. Thank you.