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Density of a liquid above a column of water

  1. Dec 29, 2015 #1
    1. The problem statement, all variables and given/known data
    A column of water 40 cm high supports a 30 cm column of an unknown liquid.What is the density of the liquid?

    2. Relevant equations


    3. The attempt at a solution
    As per the instruction of the teacher I equated their pressures.
    h1ρ1g=h2ρ2g. And hence arrived at the answer. But I don't understand why we are doing this.Why should the pressures of both the columns be equal? I am trying to think of what would happen if they weren't equal but I can't get it. The unknown liquid column will be acted on by atmospheric pressure at the top. What will the pressure at the interface of the 2 liquids be?
     
  2. jcsd
  3. Dec 29, 2015 #2

    haruspex

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    Where the two meet, right?
    If the pressure in one is P and the cross sectional area where they meet is A then what force does it exert on the other fluid? What will happen if they push on the interface area with unequal forces?
     
  4. Dec 29, 2015 #3
    Any total pressure difference that not satisfy Bernoulli's equation make fluid acceleration. For unmoved liquid, pressure on the same label must be the same.
     
  5. Dec 29, 2015 #4
    ok. So for the columns to remain in equilibrium, the pressures(or forces) they exert on each other should be equal.
    the first column exerts pressure on the second at the interface, but i thought the second only exerts pressure on the bottom of the container? but it actually does on the first.
     
  6. Dec 29, 2015 #5

    haruspex

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    They must exert pressure on each other at the interface. Action and reaction.
     
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