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Homework Help: Water rocket force

  1. Mar 6, 2004 #1

    I understand that the force with which a water rocket is accelerated is equal to the force of the reaction mass (or water) sent in the other direction.

    If the change in mass of the water as it is being ejected can be calculated from this equation:

    change of M = area of exit x water density x (velocity of water x time)

    How am I supposed to find out the change of mass if I have no idea what the velocity and time is?

    Do I have to compare that with the velocity of the rocket (which I know)?

  2. jcsd
  3. Mar 6, 2004 #2
    OK this is what I have now:

    F = delta(mv) / delta t = (density x exit area x v^2)

    But, I have no idea what v is.

    I know that the acceleration of the rocket is equal to (v^2 / 2) / displacement, and so F = ma which equals the force of the reaction mass.

    Hmm I may have answered my question..

    But just so I know-

    Without knowing the velocity of the water, there is no way I can work out the force..that would depend on the air pressure and all that...correct?
  4. Mar 6, 2004 #3


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    I'm sure it has occured to you that if you take exactly the same amount of water but increase the pressure in the rocket (by pumping air in) the rocket will move faster. Unless you have some indendent way of calculating the speed with which the water leaves the rocket you will not be able to calculate the speed of the rocket.
  5. Mar 6, 2004 #4
    Thanks for your input.
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