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U-shaped tube, pressure problem, what am i doing wrong? please help

  1. Apr 2, 2009 #1
    1. The problem statement, all variables and given/known data
    What am I doing wrong in this pressure problem?
    a U-shaped tube is filled with water.
    left side area of opening is .05m^2
    right side is .08m^2
    the right side water level is .43m HIGHER than the left side, because two pistons push down on either side with different force on each side.
    the force pushing down on right side is 138N

    Q: What is the force pushing down on left side?






    2. Relevant equations

    I know that P=F/A and also P= Pnaught + rho*g*h

    3. The attempt at a solution

    so i rearrange to get F=PA
    and P=rho*g*h (because i can factor out air pressure (Pnaught) because of the pistons,
    so i now have F= rho*g*h*A
    This is wrong though, and i cant figure out why, please help! thanks

    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Apr 2, 2009
  2. jcsd
  3. Apr 2, 2009 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    You are looking for the extra pressure needed to make the side 0.43m higher
    So you need the pressure to lift 0.43m of water, this is just the static pressure at the bottom of that extra water column.

    The extra pressure on the input piston is the same as this - then to get the force just use the area - be carefu about which area you are using.
     
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