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Homework Help: Determine pressure of a gass in a piston-cylinder device

  1. Feb 22, 2009 #1
    1. The problem statement, all variables and given/known data

    A gas contained in a vertical frictionless piston-cylinder device. The piston has a mass of 4kg and a cross-sectional area of 35 cm^2. A compressed spring above the piston exerts a force of 60N on the piston. If the atmospheric pressure is 95kPa, determine the pressure inside the cylinder.

    Ans: 123400 Pa

    2. Relevant equations

    P = F/A

    P (at depth h) = density * g * h + atmospheric pressure

    3. The attempt at a solution

    Total force = (4g+60)

    P (at depth h) = ((4g+60) / (0.35)) + (95000)

    = 95284 Pa (which is not what the answer gives.)

    many thanks
  2. jcsd
  3. Feb 22, 2009 #2
    Re: Pressure

    One error, you haven't changed 35 cm^2 to m^2 properly. Try using 35x10^-4 m^2 and you get the right answer. A pretty good trick to do it if you can't remember how many "steps" you move the ,/. sign is

    cm^2 = cm x cm = (1/100)m x (1/100)m = (1/100)^2 m^2 = 10^-4 m^2
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