A friend and myself were having a conversation today just about the nature of planets in motion and orbits (Originally we were talking about getting a rocket to mars but I digress) The earth and sun, assuming no other bodies were in the solar system and the system was as it is now - earth orbiting around the sun. Assuming space was a perfect vacuum and the earth never came into contact with any other particles in space (including the solar wind etc), the earth would continue to orbit the sun....indefinately?? The sun obviously exerts a force on the earth via gravity, which changes the earths velocity (though not its speed component). If this force is constant and never ending, doesnt this represent a perpetual motion system? Or is this simply a system at equilibrium? This led us to the question of gravity. How does gravity work in terms of force and conservation of energy? It seems that gravity is able to constantly change the velocity of a body in motion. We cant reconcile how this works... Can someone please clarify this??