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How does Newton's third (3rd) law relate to a rocket blasting off from the ground?

  1. Mar 21, 2012 #1
    I am kind of confused about this. What is the action and equal and opposite reaction?
     
  2. jcsd
  3. Mar 21, 2012 #2

    arildno

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    Re: How does Newton's third (3rd) law relate to a rocket blasting off from the ground

    The rocket imparts a force on the fuel by ejecting it , the ejected fuel exerts an equal, but oppositely directed force on the rocket.
     
  4. Mar 21, 2012 #3
    Re: How does Newton's third (3rd) law relate to a rocket blasting off from the ground

    Thanks!!
     
  5. Mar 21, 2012 #4

    sophiecentaur

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    Re: How does Newton's third (3rd) law relate to a rocket blasting off from the ground

    It's all those exhaust gases emerging from the nozzles at super high speed that the rocket engines are emitting. At the back of the combustion chambers there is a high pressure that pushes up against the inside, raising the rocket, whilst the same pressure is pushing the gases out at the bottom. The momentum of all that high speed gas is equal to the momentum gained by the rocket.
    At the start of the flight, the whole process is INCREDIBLY inefficient because there's a lot of force but not much distance raised. It gets better as the rocket starts to go faster.

    PS you don't need the ground to be there for the gases to 'push against'. However, it's got to be true that, (as with a hovercraft) whilst on the ground, there will be a bit of excess pressure under the rocket, which will help to lift it the first meter or so. Above that, the gases can escape so easily that the pressure is not helped by the presence of the ground.
     
  6. Mar 21, 2012 #5
    Re: How does Newton's third (3rd) law relate to a rocket blasting off from the ground

    Thank you so much!
     
  7. Mar 22, 2012 #6

    A.T.

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    Re: How does Newton's third (3rd) law relate to a rocket blasting off from the ground

    Fun trivia: In 1920 the New York Times published an editorial claiming that rockets cannot work in space, because there is nothing to 'push against'. They retracted it only, when Apollo 11 was already on its way to the moon:

    http://www.popsci.com/military-avia...-times-nasa-youre-right-rockets-do-work-space
     
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