At 04:49 pm 18/05/2005 -0400, you wrote: > > >Stephen A. Lawrence wrote: > >> The secret is that the planet slowed down, just a little. You stole >> energy from the planet. This becomes apparent when you consider >> conservation of momentum: The spaceship's momentum vector is now >> pointing in the opposite direction, so the planet must have picked up >> the difference. > >When considering anything involving orbits and spaceships, it's worth >keeping in mind that in classical Newtonian mechanics, gravity involves >action at a distance, and the rate of propagation of the gravitational >field of a moving body is assumed to be infinite. When dealing with >things like slow spaceships on close approaches to planets, this >approximation is adequate. > >When you start dealing with very fast moving bodies or with very long >distances the propagation delay of the gravitational field becomes >important and it's necessary to use a more accurate model. And as soon >as you assume a finite propagation rate for the G-field, you also either >need to ascribe momentum and energy to the field itself, or you need to >find some other way to "make the books balance" (assuming you want to >keep conservation of momentum and energy in the model). > >Sorry -- I've wandered rather far afield here; my fingers got away from me.
I'll forgive you. It happens to all of us. <grin> Frank
