Nigel Dyer <[email protected]> wrote: And the reply is that you cant change the total angular momentum of the > system without reference to something external to said system (e.g. the > moon) so a windmill (or an RAR engine) working within the system can take > angular momentum from the earth (e.g. by spinning), but it will give it > back when it stops . . . >
Yes. The only way you can reduce the spin of the earth is to have the body taking the angular momentum not stop. For example, you launch a rocket to the east, and the rocket never returns to earth, going to Mars instead. The rocket become something external to the system. The earth slows down. If the rocket orbits and then lands back on earth, you get back the momentum. A space elevator might use the earth's spin to launch spacecraft. It would raise the outbound spacecraft above geosynchronous orbit. Think of a passenger riding a train up a space elevator. Leaving the earth, the passengers feels their weight from gravity far above the ground, gradually fading. It would be zero as the train arrives in the terminal station at geosynchronous orbit. Then the passengers would get into another train with the floor upside-down, and the ceiling facing earth. They would travel a few thousand kilometers above the terminal, toward the counter-weight that keeps the space elevator from falling to earth. At some point they would feel weight again. From there, they would board a spaceship, which is then flung into space with as much force as the person feels in weight. It would stress the tower, slightly, and tower would pull on the earth, slowing it down, slightly. - Jed
