XPost: sci.physics, sci.space.tech   
   From: mff@hypatia.unh.edu   
      
   Greg D. Moore \(Strider\)  wrote:   
   >"Matthew F Funke"  wrote:   
   >> Penguinista   wrote:   
   >> >George Kinley wrote:   
   >> >> 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?   
      
        I can see that moving a nucleus about in some desired direction can   
   be considered "pushing on it".  However, careful examination of the   
   response above -- that rockets get thrust "by pushing against the   
   propellant being thrown out throuygh the engine" -- could be read as   
   meaning that (a) the rocket is doing the pushing on the propellant *as the   
   propellant exits* to create forward momentum (which I would agree with,   
   though I might have worded it differently); or (b) that it is the   
   propellant being shoved by the rocket against previously-expelled   
   propellant that moves the rocket forward (which I wouldn't agree with).   
   Note that I didn't say that the reply above was *incorrect*, just   
   *misleading* -- it could be misinterpreted as saying something that isn't   
   quite the case.   
        Of course, I myself could have been a little less ambiguous in my   
   last sentence -- that the atomic nucleus didn't push against any   
   *previously expelled propellant*.  The rocket did shove it out the back   
   end, though, and this shove would make the rocket move forward.   
      
   >> >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.   
      
        No disagreement here.  I just wanted to avoid the notion that a   
   rocket needs to "push against" the ground, air, previously expelled   
   propellant, or any other thing that is sitting out the back end when the   
   rocket fires in order to work.  (Rockets work better in vacuum than in   
   air, in fact.)   
   --   
                                -- With Best Regards,   
                                   Matthew Funke (mff@hopper.unh.edu)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)