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Homework Help: Bernoulli's Equation and pressure differences

  1. Nov 5, 2016 #1
    1. The problem statement, all variables and given/known data
    Problem in attached image

    2. Relevant equations

    $$P_1+\frac{\rho v_1^2}{2}=P_2+\frac{\rho v_2^2}{2}$$

    3. The attempt at a solution
    I understand everything in the solution except why $$P_A-P_B=h(\rho_{Hg}-\rho)g$$ Why do we have to subtract the density of water from that of mercury instead of just taking the density of mercury??

    Any help is appreciated.
  2. jcsd
  3. Nov 5, 2016 #2


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    Homework Helper
    Gold Member
    2017 Award

    Consider starting at point B and moving down through the U-tube all the way around until you come up the left side of the U-tube to point A. Keep track of all the pressure changes.
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