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A 'What if?' question.

  1. Jun 17, 2010 #1
    What if you could build a rocket, with the exhaust "leaving" the engine at a instantaneous speed, would such a rocket with a infinite exhaust move?

    The particles leaving the engine will reach a different destination as soon as the rocket was switched on, so how could such a rocket exert a force?

    I'm kinda confused.
  2. jcsd
  3. Jun 17, 2010 #2
    Force is the change of momentum of a body. Presumably in your scenario (which is kind of impossible) the particles leaving the rocket would change the momentum of the rocket and propel it forward. Ultimately what you are describing can't happen and that makes it sort of meaningless. The velocity of the particles would be meaningless and therefore the force they exert would also be meaningless.
  4. Jun 17, 2010 #3


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    Staff: Mentor

    Non-physical assumptions will lead to nonsensical conclusions.
  5. Jun 18, 2010 #4
    I built an engine that had an exhaust speed of C.
    It wouldn't move at all. (turn your flashlight on....)
  6. Jun 18, 2010 #5


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    Science Advisor
    Gold Member
    2017 Award

    There is a well known phenomenon of 'light pressure'. The beam from your torch represents a steady change of momentum - so your torch is a (very feeble) propulsion unit.
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