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Homework Help: I dont understand this

  1. May 15, 2006 #1
    A 120 foot high dam is shaped by the symmetric function y = 2x^(4/3). The water level is at the top of the dam. Set up an integral that gives the total water pressure against the dam. Water weights 62.5 lbs per cubic foot.

    S = integral sign

    P = F / A

    but teacher's answer is

    S 62.5(120-y)*2(y/2)^(3/4)dy

    I understand the 2(y/2)^(3/4)dy part, its the area, but I dont get what is
    62.4(120-y), its seems to me that the teacher is doing P = F * A, which is wrong... if I m not wrong... please help me clear this... Final coming!!!
  2. jcsd
  3. May 15, 2006 #2

    Doc Al

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    Staff: Mentor

    What you are probably asked to find is the total force on the dam due to the water pressure. The teacher is doing F = P * A (not P = F * A, which would indeed make no sense). 62.4(120-y) gives the water pressure as a function of vertical position (measured from the bottom of the dam).
  4. May 16, 2006 #3


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    Science Advisor

    There is no such thing as "total pressure". As Doc Al said, the problem clearly is asking for "total force" against the dam. And it is correct that F= P*A.
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