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