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Homework Help: The pressure at point P

  1. Dec 8, 2012 #1


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    1. The problem statement, all variables and given/known data
    A wide vessel and a tube is connected as shown in the figure(see attachment). Water is filled inside vessel and tube. When tube is opened the water flows out as shown in the figure. The pressure at point P,at mid-point of the tube is (cross-section of the tube is very very small)

    2. Relevant equations
    Bernoulli Equation
    Equation of Continuity

    3. The attempt at a solution
    Applying Bernoulli's Equation at the top of vessel and at point P and assuming datum level to be the base of the vessel

    [itex]P_o + \rho gH+\frac{1}{2} \rho v_1 ^2 = P_p + \frac{1}{2}\rho v_2 ^2 + \rho g (-H)[/itex]

    Assuming area of cross section of vessel and tube be A and a and applying eqn of continuity

    [itex]Av_1 = av_2[/itex]

    Substituting value of v1 in prev eqn

    [itex]P_o + \rho gH+\frac{1}{2} \rho \dfrac{a^2 v_2 ^2}{A^2} = P_p + \frac{1}{2}\rho v_2 ^2 + \rho g (-H)[/itex]

    Now there comes another unknown variable v2 the value of which is unknown to me :(

    Attached Files:

  2. jcsd
  3. Dec 8, 2012 #2
    You might want to consider this as a simple siphon, and given the wording of the question, you should take the first flow rate to be zero. V1 = 0. You know the height between the surface of the water and the exit point, the density of water, atmospheric pressure, etc. The pressure at the exit is atmospheric also.

    So you calculate v2 from that. Once you have v2, you can use the equation of continuity to get the velocity at point P (hint: it's super easy 'cause water is incompressible). Then you can work out what the pressure is at point P from there.

  4. Dec 9, 2012 #3


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    Cool! That worked. Thanks for your help.
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