XPost: sci.physics, sci.space.tech   
   From: mooregr@greenms.com   
      
   "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.   
      
      
   > --   
   >                              -- With Best Regards,   
   >                                 Matthew Funke (mff@hopper.unh.edu)   
      
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