Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook