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Homework Help: A frustrating question regarding pressure, fluid and force

  1. Dec 4, 2008 #1
    1. The problem statement, all variables and given/known data

    In an experiment a student arranges for air to blow across top of an iron plate that has a large area and a thickness, d, of 2.0 mm. The air underneath the plate is at rest.

    What is the speed of air flow above which the student would observe the plate to rise? (Ignore the small buoyancy force of the air acting on the heavy plate. i.e. ignore pairgd.)

    Patmosphere = 101300 Pa
    Density of air = 1.29 kg/m3
    g = 9.81 m/s2

    2. Relevant equations
    P + 1/2 pv2 + pgh = constant

    3. The attempt at a solution
    The answer is apparently v = 15.4 m/s.

    But I do not know how to even start this question as only little data is given.

    Your help will be very appreciated. Or any suggestions/ideas/hint would help a lot as well.
  2. jcsd
  3. Dec 4, 2008 #2


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    Homework Helper

    You have to assume the density of iron plate.
  4. Dec 4, 2008 #3
    Thanks for your reply.

    But even if I know the density of the iron plate, I am not given the area of the plate, so it won't be much of a use, no?
  5. Dec 4, 2008 #4


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    Homework Helper

    To find the pressure area is not needed. Density is enough.

    p(iron)dg = 1/2*p(air)*v^2. Try this.
  6. Dec 4, 2008 #5
    Yes you are right.
    Thank you so much.
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