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Homework Help: Fluids-blow across top of straw to draw up olive oil

  1. Dec 12, 2009 #1
    1. The problem statement, all variables and given/known data

    You try to remove olive oil from a glass by blowing across the top of a vertical straw immersed in the olive oil. What is the minimum speed you must give the air at the top of the straw to draw olive oil upward through a height of 1.5 cm?
    (Note: The density of olive oil, if you need it is 920 kg per cubic meter.)

    2. Relevant equations

    P1 + .5*density*v1^2 + density*g*y1 = P2 + .5*density*v2^2 + density*g*y2

    3. The attempt at a solution

    P1 + .5*density*v1^2 + density*g*y1 = P2 + .5*density*v2^2 + density*g*y2
    P1 = .5*density*v2^2 + density*g*y2
    V2 = 14.83 m/s
    V2 is close, but not right; I'm not sure how to find P2. What do I need to do different?
  2. jcsd
  3. Dec 12, 2009 #2


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    Homework Helper

    I don't understand your calculations. How did you get from P1 = .5*density*v2^2 + density*g*y2 to v2=14.83 m/s?

    You can calculate the pressure difference between the top and bottom of the straw that's needed to support the olive oil column; it's just rho*g*h. Then, you can apply the equation P1 + .5*density*v1^2 + density*g*y1 = P2 + .5*density*v2^2 + density*g*y2 with the left side representing the bottom of the straw and the right side representing the top. v1 would be 0, since the air at the bottom isn't moving, and you've just calculated P1-P2.
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