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Homework Help: Fluids at Rest: U-tube model

  1. Sep 13, 2009 #1
    1. The problem statement, all variables and given/known data

    The plastic tube has a cross-sectional area of 5.00 cm². The tube is filled with water until the short arm (of length d = 0.800 m) is full. Then the short arm is sealed and more water is gradually poured into the long arm. If the seal will pop off when the force on it exceds 9.80 N, what total height of the water in the long arm will put the seal on the verge of popping?

    2. Relevant equations

    P = Po + pgh
    P = F / A

    3. The attempt at a solution
    Well I know that at their equal height, their pressure is in equilibrium (equal at that point).

    H = h + d
    Fpo = 9.80 N

    (From free body diagram):
    F2 - F1 = Fpo
    PtA - PoA = Fpo

    Pt = Po + pgh

    This is where I'm stuck. I don't know what to do from here. Maybe i'm doing it completely wrong or there's an easier way. Can someone please help!

    Attached Files:

  2. jcsd
  3. Sep 19, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Don't know if you're still working on this one, but here are my thoughts.

    When the short tube is first sealed off, there is zero net force acting on the seal because the pressure is Po both above it (from the air) and below it (from the water).

    So the net force on the seal can be thought of as entirely due to the water added afterwards.
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