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Im having trouble with a simple question (simple thermodynamics)

  1. Apr 22, 2006 #1
    HI! i was wondering if someone could please help me with this physics problem. it looks very simple, and im only missing one value. thanks


    gas pressure is: p= F/A = (mg)/A(of the cylinder)

    my problem is: how can i get the mass of the piston? its really annoying me. please help. thanks
  2. jcsd
  3. Apr 22, 2006 #2

    Andrew Mason

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    What is the volume and density of the piston? (ie. what is the density of copper?).

  4. Apr 22, 2006 #3
    thanks for responding!!!

    the density is: 8.96 g/cm^3

    the volume of the piston is: [(3.14) x (.03)^2].04 = .000113 m^3 = 113.0973 cm^3

    so then, i need to multiply : M= 113.0973 x 8.96 = 1013.35212g

    The area of the container is: A= [(3.14 x .03^2)] = .002827

    So then: p = Mg/A = 3512858.979

    this is wrong though.... can you plesae help me find what i did wrong :grumpy:
    Last edited: Apr 22, 2006
  5. Apr 22, 2006 #4

    Andrew Mason

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    First of all, work out the solution algebraically and then plug in numbers. Second, you should stick to significant figures.

    Mass of piston = [itex] \rho V = \rho\pi r^2h[/itex]
    Pressure = Mg/A = [itex]\rho\pi r^2hg/\pi r^2 = \rho hg[/itex]

    g = 9.80 m/sec^2
    [itex]\rho = 8.96 g/cm^3 = 8.96 x 10^-3 kg/10^{-6} m^3=8.96 x 10^3 kg/m^3[/itex]

    Pressure = 8960 x .04 (9.80) = 3512 N/m^2 = 3512 Pa. = 3.512 kpa

    In significant figures, this would be 3.5 kpa

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