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Homework Help: Question about water weight and pressure

  1. Sep 18, 2008 #1
    So the question asks:

    A circular wine barrel 75 cm in diameter will burst if the net upward force exerted on the top of the barrel is 643 N. A tube 1.0 cm in diameter extends into the barrel through a hole in the top. Initially the barrel is filled to the top and the tube is empty above that level. What weight of water must be poured into the tube to burst the barrel?

    I understand that the initially pressure in the tube is Patm before any water has been poured into it and that that pressure will later become P = Patm + Pwater(g)(h) but I can't seem to figure out how to answer the question.

    Any help would be appreciated!
  2. jcsd
  3. Sep 18, 2008 #2


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    Staff: Mentor

    Moved to homework help....

    Use the pressure vs force equation: p=f/a to relate the force on the top of the barrel to the pressure of water in the tube.
  4. Sep 18, 2008 #3
    I never done anything with pressures so I might be wrong, but this is what I did:

    Area of small tube = 7.85e-5m^2
    Area of barrel = 0.446^2


    So P1= 1441 Pa

    Then I solved for F1, getting 0.113N which divided by 9.8 gives .0115Kg of water.
    I omitted the atmospheric pressure because I have no clue how to use it.

    And again, first time I even see the formula for Pressure so I'm possibly wrong (my final answer is pretty weird to be real).
    Last edited: Sep 18, 2008
  5. Sep 18, 2008 #4
    i tried to relate those formulas but i don't know what i am supposed to use as a value for A
  6. Oct 3, 2008 #5


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    Homework Helper

    Hi lyndan! :smile:

    You've probably worked this out by now,

    but in case you haven't, you'll need to use A twice …

    once for the cross-section area of the tube, and once for the cross-section area of the barrel. :wink:
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