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Homework Help: Given a piston find pressure

  1. Nov 6, 2014 #1
    1. The problem statement, all variables and given/known data
    Find the gauge pressure of a piston-cylinder system, assuming that the piston perfectly covers the cylinder. The piston's radius is 5 cm, its mass is 3kg, and the force being applied to it is 75N in the same direction as gravity. Looks like this picture: http://www.ohio.edu/mechanical/thermo/Intro/Chapt.1_6/energy_eqns/energy_eqn.gif
    and the force is downwards on the pistom.

    2. Relevant equations

    3. The attempt at a solution
    The answer depends on whether or not the mass should be taken in to account.

    Ignoring piston's mass:
    P= 75N/(π(5cm/100)^2)=9.54e3 Pa = 9.54 kPa

    Accounting for piston's mass:
    P =((ma)+75N/(π(5cm/100)^2)) = ([(3kg)(9.81m/s/s]+75N)/(π(5cm/100)^2) = 13.3e3 Pa = 13.3 kPA
  2. jcsd
  3. Nov 6, 2014 #2
    So what's the force per unit area being exerted on the gas in the cylinder (at the lower face of the piston)?

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