On Earth there is always air or water available to push against.
Ah. I see. I interpreted that statement as simply referring to the fact that the propellor blades transfer momentum to molecules of air by hitting them, although I admit that "push" is probably not the best verb to describe the process.
71 posted on 02/28/2003 7:00:16 PM PST by sourcery
(The Oracle on Mount Doom)