1. The problem statement, all variables and given/known data This is my first question and I'll try to stick to the forum rules as much as possible. imgur.com/a/2JyZC Imagine a cylinder filled with air which is uniformly distributed and a piston above it, so the air can expand vertically. Let the system be the air itself, so the piston is not included. Due to a resistor in the cylinder, the volume of the air increases with 0.045m^3. The area of the piston surface is 0.09m^2. The mass of the air in the cylinder is 0.27kg, what is the change in potential energy of the air after the process? ΔV=0.045m3 Apiston=0.09m2 mair=0.27kg g=9.81m/s2 ΔPE=... 2. Relevant equations ΔPE=mair*g*Δz And I assume Δz being the change in height of the center of mass of the air in the cylinder. 3. The attempt at a solution 1) The change in volume and the area of the piston is given, so the air expands with ΔV/Apiston=(0.045/0.09)m=0.5m 2) The center of mass of the air moves with half of this calculated length, so 0.25m. 3) ΔPE=(0.27*9.81*0.25)J=0.662J According to my book: What is the change in potential energy of the air, in kJ? Ans. ≈1.055*10-3kJ Not sure where I went wrong on this one. It's almost twice my answer. Is it wrong to take the change in height of the center of mass? Thanks in advance.