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Does a rocket/spacecraft moving in space violate Newtons laws?

  1. Sep 29, 2011 #1
    How does a spacecraft move in space where there is vacuum and nothing to provide normal reaction for the motion?
  2. jcsd
  3. Sep 29, 2011 #2


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    You don't need anything to provide a reaction - that's a common misconception about rockets. Rockets work by throwing fuel out in one direction very quickly, which pushes the rocket in the other direction. They are actually more efficient in space, since there is nothing opposing the motion of the fuel.

    As for the action-reaction pair? The action is that the rocket is pushing on the exhaust, and the reaction is that the exhaust is pushing back on the rocket. No outside matter is used, nor is any necessary.
  4. Sep 29, 2011 #3


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    Actually a rocket in space would be a great example of Newton 1; objects in motion stay in motion.
  5. Sep 29, 2011 #4


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    Conservation of momentum.

    You're sitting on a rocket stationary in space.

    Throw a rock out the back of the rocket, and the sum of rock's momentum and the rocket's momentum still stays constant (0 in this case). That means that the mass times the velocity of rock has to equal the mass times the velocity of the rocket. Naturally, since the rocket is so much bigger, it's velocity will be a lot less than the rock's.

    To get any kind of significant change in the rocket's speed, you need to increase the total momentum of stuff you're throwing out the back. Either throw a whole lot of rocks out the back (which would have the added benefit of reducing the rocket's mass) or throw the rocks out the back at incredibly high velocities.

    Rocket engines are essentially throwing fuel out the back at incredibly high velocities. And the fuel is usually some combination of chemicals, since the chemical reaction is the method of getting that fuel to fly out the back really fast.
  6. Sep 29, 2011 #5
    If you're referring to how spaceship movements are depicted in movies (i.e. like fighter planes, swooping and turning and such despite being in vacuum), yes, Hollywood typically ignores basic physics for the sake of entertainment.
  7. Sep 30, 2011 #6
    Thnx, convinced...
  8. Sep 30, 2011 #7


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    Not a fuel - reaction (which in many cases can be classified as combustion) products. Unless you mean something like ion thruster.
  9. Sep 30, 2011 #8

    D H

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    It's not just Hollywood. The media is quite ignorant of science in general. Witness the recent reentry of the UARS space craft. It didn't matter which news outlet I watched; they all got the basic physics wrong. There was an implicit (and sometimes explicit) assumption on the part of the newsie that in order for a spacecraft to be orbiting the Earth it must be firing its thrusters constantly. Stop firing the thrusters and the satellite will stop orbiting. Immediately. Some announcers just couldn't understand why NASA couldn't control the reentry better.

    What actually happened with UARS is that NASA intentionally gave the vehicle its last thruster firing in December, 2005. This decommissioning burn lowered the perigee by almost 200 km, putting the perigee well within the outer atmosphere. It was atmospheric drag that caused the vehicle's final demise. If the Earth had no atmosphere that vehicle would still be orbiting today, even with that decommissioning burn.
  10. Sep 30, 2011 #9


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    Fair enough. I was intentionally simplifying it, and that statement is possibly misleading. I should have said that they throw propellant or reaction mass out at high speed.
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