XPost: sci.physics, sci.space.tech   
   From: glhansen@steel.ucs.indiana.edu   
      
   In article ,   
   Greg D. Moore \(Strider\)  wrote:   
   >   
   >"Matthew F Funke"  wrote in message   
   >news:beu93v$r9p$1@tabloid.unh.edu...   
   >> 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."   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)