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Thrust; the very basics

  1. Aug 13, 2015 #1
    I read this description of thrust on gcsescience.com;

    "Hot gases are forced downwards through the rocket's jets
    which pushes the body of the rocket upwards.This is an example of Newton's Third Law of Motion. "

    I've learned already that Newton's 3rd law states that for every action there is an equal and opposite reaction, eg, when I push against a wall, the wall pushes against me (if it didn't I would end up pushing the wall away). I don't find this easy to grasp fully yet but it seems to make some sort of sense. But in the rocket example above, if I relate that to the example of me pushing against the wall, who is me and who is the wall?
  2. jcsd
  3. Aug 13, 2015 #2
    1. Rockets take off by burning fuel. Burning fuel produces gas as a byproduct, which escapes the rocket with a lot of force. The force of the gas escaping provides enough thrust to power the rocket upwards and escape the the force of gravity pulling it back to Earth.
  4. Aug 13, 2015 #3


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    The rocket and it's exhaust gases push against each other.
  5. Aug 13, 2015 #4


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    Newton's third law says that even if you succeed in pushing the wall down, the force the wall exerts on your hands is still equal and opposite to the force of your hands on the wall.

    In the case of the rocket, the hot gases are the wall. The rocket succeeds quite well in pushing the wall down and flinging the pieces far away.
  6. Aug 14, 2015 #5
    Thanks. So, if the hot gases are the wall, the rocket is me, yes? Does the Earth and the air beneath the rocket play no part in this action and equal and opposite reaction scenario; is it only between the rocket and the hot gases it expels?
  7. Aug 14, 2015 #6
    Is this similar to two roller skaters pushing each other away from one and another, ie, one of them being the rocket, the other the hot gases?
  8. Aug 14, 2015 #7
    How , do you feel , would the Earth affect the force between the rocket and the gases ?

    Also , air in space ? :biggrin:
    Last edited: Aug 14, 2015
  9. Aug 14, 2015 #8
    Post edited .
  10. Aug 14, 2015 #9
    My guess is that the Earth wouldn't affect the force between the rocket and the gases, it would only affect the force required to accelerate the rocket away from the Earth.
    As for air in space; I was referring to take-off and the rocket's subsequent acceleration away from the Earth where there would be air beneath the rocket.
  11. Aug 14, 2015 #10


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    That is correct on both counts. The Earth has no effect on the force between rocket and the exhaust gasses. It only affects the force between rocket and Earth.
  12. Aug 15, 2015 #11
  13. Aug 17, 2015 #12


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    Either way works. Either you are the rocket pushing the gas (the wall) out the nozzle and you get thrown forward, or you are the gas being pushed out the nozzle and the rocket is the wall going forward.
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