Why isn't weight included in piston force equations?

[eurekameh]

http://imageshack.us/photo/my-images/40/fdsfsadf.png/
So here's an image of a piston. First of all, this is for a thermodynamics class. Is thermodynamics considered to be "physics?" Do physics majors take thermodynamics?

So my question is: When writing the forces on the piston, it is obvious that we have the pressures acting on the piston from the gases in the chambers, and also a spring force on the second piston figure. I thought it was also obvious that there is Fg = mg acting on the piston as well. However, when I peeked at the solution manual, weight is not included in the force equation. Why is this?

 

[rl.bhat]

Post the exact text of the problem.

 

[eurekameh]

The piston diameters in Figure are D1 = 10 cm and D2 = 4 cm. When the pressure in chamber 2 is 2000 kPa and the pressure in chamber 3 is 700 kPa, what is the pressure in chamber 1, in kPa?