"do you mean as in gravity assists? expansion explains that. gravity does not. there is no such thing happening via a gravitational force. there is no momentum transfer. for example, a craft accelerated in gravity assist would, then, be slowed on it's path of departure from the vicinity of the planet, as the planet pulled back on the craft. the craft would not undergo a net speed increase via gravity assist. yet this logic is never questioned. conversely, it has become an axiom and matter of fact. when, really, it is a fantasy."<br /><br /><br />I was a little confused at this and stayed from replying, but then re-reading some of the thread here it now makes more sense.<br /><br />in gravity assist and 'slingshot', which most everyone here knows is a very real phenomena used by NASA with their satellites, the craft, in approaching a planet, has enough velocity to overcome an orbit trajectory, and can thereby use the planets gravity to further accelerate it.<br /><br />Planets however do not have enough velocity to overcome the Sun's attractive force and thereby, due to the inverse square law, the sun will eventually slow the planet down (where planet eventually reaches the apex of the second focii in orbit- forget the term used...).<br /><br />since we are dealing with varying speed, kind of proves the inverse square law at work for a force. it takes more time to eventually pull the planet back, directly related to speed and inversely to its distance.<br /><br />the craft in a trajectory already has enough velocity to overcome the orbit (at whatever distance) and hence the slingshot effect. As the planet pulls on it, it accelerates even more, the planet has even less pull on its outward trajectory and the craft not only escapes crashing in or finding orbit, but has additional velocity tacked on--- momentum transfer. yes yes yes.<br /><br />we also note this subatomically with electrons and their orbits around a nucleus. free electrons will use atoms close by to bounce off (s <div class="Discussion_UserSignature"> </div>