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Homework Help: Help Physics Fluids problem

  1. Dec 4, 2006 #1
    Help!! Physics Fluids problem

    1. The problem statement, all variables and given/known data
    Water is forced out of a fire extinguisher by air pressure. How much gauge air pressure in the tank (above atmospheric) is required for the water jet to have a speed of 29.8 m/s when the water level is d=0.515 m below the nozzle?

    2. Relevant equations
    I tried using bernoulli's equation, p1+ 1/2*p*v1^2 + p*g*y1= p2 + 1/2*p*v2^2 + p*g*y2
    I am not sure what variables I am supposed to put in where, as the diagram i am given in the question is that of a tank with a water pipe in the middle of it.

    3. The attempt at a solution
    I attempted to solve for p2, using p1 as atmospheric pressure, density as the density of water, v1 as 29.8m/s^2, height as 0.515. The rest of the terms on the second half of the equation I thought would cancel because the height would be 0 and I thought the starting velocity is 0. I ended up getting -4.49 x 10^5 Pa...which isn't right...please help!
  2. jcsd
  3. Dec 5, 2006 #2


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    Science Advisor
    Homework Helper

    The height of the water in the jet is below the nozzle. I think you want to either make y1 negative, or make it zero and use the .515m for y2.
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