In article ,
Greg D. Moore \(Strider\) wrote:

"Matthew F Funke" wrote in message
...
Penguinista wrote:
George Kinley wrote:
Hi,
if there is no Atmosphere, where do rockets that go in Space get thrust
from


Goddard was pestered with the same question. The answer is simple, by
pushing against the propellent being thrown out through the engine.

This is misleading. Let's say you had a very tiny rocket in a
perfect vacuum, and hurled *one atomic nucleus* out of the back end at
extremely high velocity. The rocket would still be propelled in the other
direction, even though the atomic nucleus didn't push against anything,
since the momentum of the system would have to be conserved.

How do you hurl the atomic nucleus out of the back of the rocket w/o somehow
pushing on it?



Consider a kid on a very low drag sled and a pile on beanbags. By
throwing the beanbags in one direction, he can build up speed in the
other direction.

Note that this beanbag system would also propel the kid in a perfect
vacuum, even if the beanbags he threw never collided with (or "pushed
against") each other.

But again, the kid is pushing on the beanbags. That's the key. For every
reaction there's an equal and opposite reaction. There's no way to make the
beanbag go in one direction w/o pushing on it somehow.

F=dp/dt, so p=\int F*dt. Push hard for a short time, push softly for a
long time. Either way the problem can be turned to one of conservation of
momentum and you don't have to worry about the details of the push.
Universality is one of the great things about conservation laws.

--
"When fighting with sharpened Bronze, or harder Metals from the Heavens,
it is Wise to kick thy Opponent, be he a Chaldean or a man of Uruk, in his
Man Sack, that thou mayst defeat him more handily than by Arms. So sayeth
INNAMURUTUSHIMMILODEK, who hath slain threescore Ammelekites."