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Homework Help: A blowhard.

  1. Apr 5, 2007 #1
    1. The problem statement, all variables and given/known data
    Tests of lung capacity show that adults are able to exhale 1.5 liters of air through their mouths in as little as 1.0 seconds. a)if a person blows air at this rate through a drinking straw with a diameter of 0.60 cm, what is the speed of air in the straw? b) if the air from the straw in part (a) is directer horizontally across the upper end of a second straw that is vertical in water, to what height does water rise in the vertical straw.

    2. Relevant equations

    3. The attempt at a solution
    i have found the answer to part (a) and it is 53 m/s and it is correct. however, i have no idea how to approach b. any help would be great.
  2. jcsd
  3. Apr 5, 2007 #2
    i think you need to use Bernoulli's eqn. The velocity of the airstream across the other straw will reduce the ambient pressure.
  4. Apr 5, 2007 #3
    i tried that, but im not sure exactly how it works. i got something like 140 cm when the answer is 19 cm.
  5. Apr 5, 2007 #4


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    Perhaps if you showed your work, we could give you a hand...
  6. Feb 26, 2008 #5
    for a) i got the radius of straw r=.003 m
    area and 1.5 L is .0015 m^3. Divide cubic meters by area of straw, and got 53.1 m/s

    But for part b) How do you use Bernoulli's eqn? Do you assume the pressure at the top of the straw is appoxamately zero, since the air is being blown off?
  7. Feb 26, 2008 #6
    When I assume that, I get a little over 10 meters... and that can't be right.
  8. Feb 26, 2008 #7


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    No, Bernoulli's law connects the speed of air across the straw with the pressure. Use Bernoulli's law to calculate the pressure.
  9. Feb 26, 2008 #8
    so do I use the equation twice?
    101.3 kPa = Pressure + (.5*1.29*53*53)

    Pressure = 99488 Pascals

    and then plug that in using water's density as rho the next time?
  10. Feb 26, 2008 #9
    I got the difference in pressure as being 1812 Pacals.

    so that equals 1000kg/m3 * 9.8 m/s * delta h ?

    the height is .1848 meters?
  11. Feb 26, 2008 #10
    That doesn't make sense still.
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