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Homework Help: Rocket Problem - Law Of Conservation Of Momentum

  1. Feb 2, 2005 #1

    I am unable to solve this problem. :zzz:

    I applied Law of Conservation of Momentum but unable to get the related answer.

    The problem is as. :!!)

    "A rocket is moving vertically upward. % Kg fuel gas is released out of it in downward direction with a velocity of 5000 m/s. If the mass of rocket is 1000 Kg, what will be its velocity?"

    The answer is –25 m/s.


    Adabistan-e-Soophai :cool:
  2. jcsd
  3. Feb 2, 2005 #2


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    Since the momentum of the fuel that the rocket spits out is (velocity times mass): 5000*5=25000 kgm/s. (I take it the % is a 5).

    Then since total momentum is conserved, the rocket gains a momentum of 25000 kgm/s in the opposite direction. Use the momentum equation for the rocket to get it's velocity.

    This question does assume the rocket has zero velocity to start with (so you're viewing from the rocket's initial frame of reference).
    Also, the ejection assumed to be instantanuous. That is, a big clumb of fuel is spit out at an instant. If the ejection was gradual (as in reality) the velocity would be different, because the mass of the fuel had to be incorperated into the mass of the rocket.
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