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Homework Help: Hydrostatic forces

  1. Dec 20, 2009 #1
    I need to find the hydrostatic force exerted on a plane submerged vertically in water. I attached a diagram of the problem.

    Here are the basic definitions:
    ---------------------------
    d=distance from surface, p=density, P=pressure

    [tex]p=\frac{m}{V}[/tex]

    [tex]P=pgd=\delta d[/tex]

    [tex]F=mg=pgAd[/tex]

    ---------------------------

    The area of the ith strip is [tex]A_i=6\Delta y[/tex] so the pressure exerted on the ith strip is [tex]\delta d_i=pgd_i=pg(6-y_i^*)[/tex]

    The hydrostatic force on the ith strip is [tex]F_i=\delta_iA_i=6pg(6-y_i)\Deltay[/tex]

    The approximate force along the entire surface is therefore:

    [tex]F_{net}=\lim_{n-\infty}\Sigma_{i=1}^n6pg(6-y_i)\Delta y[/tex]

    [tex]=6pg\int_0^4(6-y)dy[/tex]

    Am I setting this up correctly?
     

    Attached Files:

  2. jcsd
  3. Dec 20, 2009 #2

    rock.freak667

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    It looks like you are doing it correctly from first principles, but I think this line should be (not sure on notation but this is how I saw a similar summation in a math book)


    [tex]F_{net}=\lim_{\Delta y \rightarrow 0} \sum_{y=0} ^{y=4} 6pg(6-y_i)\Delta y[/tex]
     
  4. Dec 20, 2009 #3
    I was writing out the limit of the Riemann sum. There are [tex]n[/tex] subdivisions and [tex]\Delta y=\frac{4-0}{n}[/tex]. So I think what you wrote was equivalent to the Riemann sum.
     
  5. Dec 20, 2009 #4

    rock.freak667

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    It probably is, I was never taught the Riemann Sum, but you are correct though.
     
  6. Dec 20, 2009 #5

    ideasrule

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    You don't need to write out the Riemann sum; that just makes things unnecessarily complicated. I find that going directly to Fnet[itex]=6pg\int_0^4(6-y)dy[/itex] is much easier and more intuitive. (BTW, that integral gives the exact force on the plate, not the approximate force.)
     
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