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Homework Help: Fluid mechanics help, where am i going wrong?

  1. Aug 28, 2011 #1
    1. The problem statement, all variables and given/known data
    [PLAIN]http://img30.imageshack.us/img30/6918/screenshot20110828at111.png [Broken]

    2. Relevant equations
    P_base= rho * h * g = 1800 kg/m^3 * h * 9.81 m/s^2 = 17658 * h kg/(m^2 s^2)
    F_h= .5 * h * P_base = 10594.8 * h^2 kg/(m^2 s^2)

    3. The attempt at a solution
    I am trying to find the force F_h by calculating the pressure at the bottom of the load and creating a resultant. The issue i believe i am having is that there is no depth given so I'm not sure how to calculate the force on a wall since i only have 1 of the walls dimension. Unless I'm reading the problem incorrectly and the width and depth are both .25m? When i do the math the F_h works out to be kg/s^2 instead of newtons.

    Once i figure out the F_h i would then just need to take the moment about A to find the tipping point and the max F_friction to find the highest sliding point, correct? If anyone could guide me as to what I'm doing wrong that would be helpful.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 2, 2011 #2

    rude man

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    Homework Helper
    Gold Member

    I believe you're on the right track.

    When a dimension is omitted in a problem you can be pretty sure it's not needed. Just assume a 1m depth. Then compute the resultant for F_H to get the slip point and sum torques (moments) about A as you say.

    Isn't it funny how they say "retaining wall" instead of just "dam"?

    PS - force is now pressure * effective height * 1m, you realize.
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