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Fluid dynamics: Knowledge continuity equation

  1. Sep 24, 2012 #1
    1. The problem statement, all variables and given/known data
    Please click on the link for the question.


    Assume that air spreads out after coming out from the tube at 2. The speed over tube 1 is almost zero.

    2. Relevant equations

    Knowledge problem on fluid dynamics
    Continuity equation A1v1 = A2v2

    3. The attempt at a solution

    I know that the higher the liquid height of the tubes, the lower the air pressure. Higher flow speed also results in low pressure. So when the cross sectional area is large, the flow speed should be slow.

    Using this reasoning, my answer was h2 > h4 > h3 > h1 but that answer is wrong.

    Is something wrong with my logic?

    Any help would be appreciated.
  2. jcsd
  3. Sep 27, 2012 #2


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    I agree with your logic.

    The fact that the author gave two options where h3>h4>h2, and leaves you to think about how the open-air height h1 relates to those in the tube, suggests that the author might have it backwards. Perhaps he or she was thinking about pressure, which has the reverse relation as the heights, i.e. p1>p3>p4>p2
  4. Sep 27, 2012 #3


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    Hi again,

    I have discussed this problem with some of the other helpers here, and we now believe that answer is wrong as you were told.

    As a hint, think about how fast the air is moving right at the surfaces of each of the liquid columns.
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