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Homework Help: Finding Hydrostatic Force on the Plate

  1. Oct 9, 2011 #1
    Before I begin I would like to say hello to everyone. This is my first post. I will follow and abide by all rules here.
    1. The problem statement, all variables and given/known data
    A rectangular plate measuring 2m by 5m is suspended vertically in a liquid with density 500 kg per cubic meter in such a way that one of its longer sides is on top and is 3 meters below the surface of the liquid. Find the hydrostatic force on the plate.

    2. Relevant equations
    The force(F) equals = Area*depth*density*acceleration

    3. The attempt at a solution
    F= A * depth * density * acceleration
    The density and acceleration are 500 * 9.8 = 4900, so that goes outside the ∫
    The bounds of the integral are 3 to 5 since the plate is 3 meters deep.
    The Area I think is equal to 5 * x and the depth I think is (3-x)

    So my final integral would be 4900 ∫5x (3-x) dx from 3 to 5

    Not sure if I'm right or not
  2. jcsd
  3. Oct 9, 2011 #2


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    Welcome to the forums! When you are integrating you are basically summing over small rectangular slices of the area. I think you mean x to be the depth of that slice. Picture a slice of your plate going from x to x+dx in depth. That makes the area of that slice dx * width of the plate, or 5*dx, right? And depth of the slice is just x. Try and set that integral up again.
  4. Oct 9, 2011 #3
    I decided to set up the integral differently so now I have:
    4900∫5(3+x)dx from 0 to 2
    =196000 Newtons

    (3+x) is the depth
    4900 is the density*acceleration due to gravity(both are givens)

    or I could have done

    4900∫5x dx from 3 to 5

    Helped me realize I would get the same answer.
  5. Oct 9, 2011 #4


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    Sure. Good job!
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