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Homework Help: Drinking through straw

  1. Dec 20, 2007 #1
    1. The problem statement, all variables and given/known data

    A drinking straw 20cm long and 3.0mm in diameter stands vertically in a cup of juice 8.0cm in diameter. A section of straw 6.5cm long extends above the juice. A child sucks on the straw, and the level of juice in the glass begins dropping at 20cm/s.

    (a) by how much does the pressure in the child's mouth differ from atmospheric pressure?

    (b) What is the greatest height from which the child could drink, assuming the same mouth pressure?

    2. Relevant equations

    P = P_o + [tex]\rho[/tex]gh

    p + [tex]\rho[/tex]gh + [tex]\frac{1}{2}[/tex][tex]\rho[/tex]v[tex]^{2}[/tex]

    3. The attempt at a solution

    I'm just reviewing for my final exam. Am i in the right direction, and if yes, guide me a little. Thanks
  2. jcsd
  3. Dec 21, 2007 #2

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    Equate the quantities in your 2nd expression for the values at the surface of the liquid and at the point where the liquid exits the straw, viz., the mouth. Take h as the height from some reference level, e.g., the bottom of the straw.
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