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Homework Help: Fluid Motion

  1. Nov 15, 2006 #1
    Ok, Air flows through this tube at a rate of 1200 cm^3/s. Assume that air is an ideal fluid.
    What is the height h of mercury in the right side of the U-tube?
    Here's a picture:

    First, I found the velocity of the air in the 2 cm and 4 mm tube.
    In m/s, for the 2cm, i came up with 1200/(1000000pi(.01^2))
    and for the 4mm 1200/(1000000pi(.002^2))
    Then, I plugged it into bernoulli's equation, with the density of air as 1.2 kg/m^3. This gave the 2cm tunnel to have 5462.5 less pascals than the other 4 mm tube.
    Then, using 13540 for the kg/m^3 of Hg, and setting the pressures of the two sides equal, I came up with:
    5462.5 = 13540 (9.8) (h), which gave me h= .0412 m, or 4.12 cm, which is wrong. Anybody know what I did wrong?
  2. jcsd
  3. Nov 16, 2006 #2


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    I'm not seeing a problem with what you did. Do you know the answer that is expected?
  4. Nov 16, 2006 #3
    I wish I knew the answer that is expected :P Do you know if those are the right densities I used in kg/m^3?
  5. Nov 16, 2006 #4
    Found out I was using the wrong densities ^^ I used 1.3 kg/m^3 for air and 13700 for Hg and got the right answer. My physics book is too big to bring home, so I rely on google for my constants :(
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