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Homework Help: Horizontal Force in Pipe Bending

  1. Mar 20, 2012 #1
    1. The problem statement, all variables and given/known data
    Water enters a pipe and goes through a bend with a contraction from 40mm to 25mm then into the atmosphere.
    What is the horizontal force required to hold the pipe in place?

    2. Relevant equations
    I have either been given the following or have calculated them using the standard formula, I am confident that they are correct.

    Q = 0.01m3s-1
    A1 = 1.26 x 10-3m2
    A2 = 4.91 x 10-4m2
    u1 = 7.94ms-1
    u2 = 20.37ms-1
    p1 = 31.52Pa
    p2 = 207.47Pa
    θ = 60°
    ρ = 1kgm-3
    m = u1A1ρ = 0.0100044

    3. The attempt at a solution
    Fx = p2A2cosθ-p1A1-m(u2cosθ-u1)

    Using the above equation that I found from a few online sources I get the result:

    So does this mean I multiply my answer my -1 to make it positive and this is the final result? Or have I gone wrong in my equation?

    Any help is appreciated,

  2. jcsd
  3. Mar 23, 2012 #2
    You should always draw an sketch of the situation and place a force, Fx, on it. Then do a force balance to determine Fx. If Fx < 0, it will actually point in the opposite direction from the way you drew it.

    In your case if you drew the sketch, Fx would be pointing in the direction of the force denoted by P1A1.
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