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Pressure and force

  1. Jul 7, 2011 #1
    Hello I am currently building a water rocket. I would like some help on finding the equation for the force that is exerted when I add air pressure to an cylindrical tube filled with water. How can I figure out the force that the water would create when I release it. I hope that someone can help with this problem of mine. I appreciate the help that anyone can offer me.

    Thank you,
    Dru Jouhnson
  2. jcsd
  3. Jul 8, 2011 #2
    For a rocket, the force (thrust) generated is given by:

    Thrust = mdot * v_exit


    mdot = mass flow rate
    v_exit = velocity of fluid at exit of nozzle

    For a water rocket pressurized with air, you could figure out the mass flow and velocity from fluid mechanics. As the water is expelled, the gas expands, pressure drops, flow rate drops, and the thrust drops. Thus, the thrust decreases with time. But the weight of the rocket decreases too, so the acceleration may increase or decrease depending on the variables.
  4. Jul 8, 2011 #3
    Does the nozzle of the rocket act like the throttle valve in a refrigerator? The pressure of the refrigerant is sharply reduced by the throttle valve which also reduces the temperature of the refrigerant.

  5. Jul 8, 2011 #4


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    Not exactly. Since the water is basically incompressible, the temperature will largely be unaffected by the water's travel through the nozzle. The pressure will drop dramatically with a corresponding increase in velocity though (as a rough estimate, you could assume that it is an ideal nozzle, and thus the water's exit velocity is equal to sqrt(2P/rho)).
  6. Jul 8, 2011 #5
    Are liquids typically incompressible and gases typically compressible?

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