How Is Gauge Pressure Calculated in a Piston-Cylinder System?

[raddian]

Homework Statement

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.

Homework Equations

P=F/A
F=ma

The Attempt at a Solution

The answer depends on whether or not the mass should be taken into 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

 

[Chestermiller]

So what's the force per unit area being exerted on the gas in the cylinder (at the lower face of the piston)?

Chet