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Homework Help: Fluid pressure (verification please)

  1. Dec 18, 2011 #1
    1. The water in a tank is at a gauge pressure of 267,018 Pa. If the bottom of the tank that has an area of 0.8 m2, what is the force on the bottom of the tank?

    2. P=F/A; F=P*A

    3. Converted Pa into 38.8psi for P (267,018/6890); Converted the Area into 103.7 in^2 (.8*10.8*12); Multiply [P*A] 38.8 * 103.7 = 4023.6 in lbs = 335.3 lbs of force. Did I do this right?
  2. jcsd
  3. Dec 18, 2011 #2
    The portion about the Paschals to PSI is pretty close but the area portion is wrong.
  4. Dec 18, 2011 #3
    We are to use all units in the book and to 1 decimal place rounded up so that's where I came up with the Pa conversion.

    Hmmm, in my book, the conversion is 1m^2 = 10.8ft^2, so I converted the m - ft by multiplying it leaving 8.6ft^2. I had to get the units to be the same so converted ft^2 to in^2, multiplying it by 12. Where did I go wrong? Thanks in advance for your guidance.
  5. Dec 18, 2011 #4
    How many SQUARE inches in a SQUARE foot?
  6. Dec 18, 2011 #5
    gotcha, I needed to multiply by 144, not 12, right? But in that case, my numbers seem way too high.
  7. Dec 18, 2011 #6
    You have pressure of roughly 38 psi with an area of .8 m^2 which is about 1240 in^2. The force is large.
  8. Dec 18, 2011 #7
    So that leave me 48,275 psi, or 335.2 ft lb^2. The force is 112,359 lbs.
  9. Dec 18, 2011 #8
    No. Pressure times area is force. Why do you write psi for force? Force is pounds. Pressure is pounds per square inch.

    The units work algebraically just like variables.

    (lbs/in^2)X(in^2) = lbs

    The pressure is 267018 Pa which is 38.73 lbs/in^2
    The area is 0.8 m^2 which is 1240 in^2.

    Multiply them!
  10. Dec 18, 2011 #9
    OK, I am totally confused. Let me start over.

    The pascal conversion was fine, no need to work with that anymore. 38.8 lb/in^2
    Area: [1m^2 = 10.8ft^2]; [10.8ft^2 = 1555.2 in^2] Convert the area 1555.2 * 0.8 = 1244.2in^2.
    The force is 38.8 * 1244.2 = 48,275lbs
  11. Dec 18, 2011 #10
    That's it. You've got it now.
  12. Dec 18, 2011 #11
    Thanks Lawrence for helping me understand what I was doing wrong.
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