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Homework Help: Pressure in U-tube

  1. Aug 25, 2009 #1
    1. The problem statement, all variables and given/known data
    Two liquids A and B of densities 1.0 g cm-3 and 0.8 g cm-3 are poured into an U-tube as shown in the figure below. Find x !


    2. Relevant equations
    [tex]P = \rho g h[/tex]

    3. The attempt at a solution
    Is it possible liquid A is on top of liquid B because A has higher density than B?

    Or do I misinterpret the question? Is it possible the question means density of A is 0.8 g cm-3 and B is 1.0 g cm-3 because there is no word "respectively" ?

  2. jcsd
  3. Aug 25, 2009 #2

    Doc Al

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    Staff: Mentor

    Sure, why not? Look at the diagram. Note that the height of liquid A is less than that of liquid B. There's a reason for that!
  4. Aug 25, 2009 #3


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    Science Advisor
    Homework Helper
    Gold Member

    The statement looks correct, A is more dense than B. At the line PQ the pressure is the same. So more height of liquid above point P is needed to maintain the pressure the same as at point A.
  5. Aug 25, 2009 #4
    Hi Doc Al and kuruman

    Oh so it's possible that liquid with higher density is on top of liquid with lower density. I always think it's impossible.

    I get it now. Thanks a lot Doc Al and kuruman ^^
  6. Aug 25, 2009 #5


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    Homework Helper

    Just imagine an iron piston pressing down on water in the U-tube and you'll see why it's possible.
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