I'm sort of confused on the boundries of it. Example. A while back, there was a post about a shuttle getting sling-shotted using Jupiter's gravitational pull. I understand that it works only because Jupiter is moving. But if we use principle relativity, from Jupiter's standpoint, it's standing still. The earth and the sun revolve around it, so from that standpoint, the shuttle should enter and leave it's pull without the excess increase or decrease of the shuttle's speed.

There was a quantum mechanical problem I had a problem dealing with, but I now forgot. If I remember it, i'll post it. It had to do with energy content and the nature of the electron and if you placed a point of reference on the electron, the something shouldn't have happened. But again, I forgot what it is.

As observed from Jupiter, the spacecraft would go back out with the same speed at which it came in, but in a different direction. If we plot those vectors and then transform them to an inertial frame of reference, we will see a difference in the speed if the directions are suitably chosen.