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?