Hi, I'm studying thermodynamics and I got a question, it's not a homework or anything, it simply isn't covered in my book and it got me thinking I know that if I have cylinder filled with gas, with a piston on top and there are weights on it maintaining an equilibrium if I remove the weights slowly the gas will expand doing work on the surroundings. After all the gas molecules will be hitting the piston wall, and as the gas pressure is higher than the external pressure it will expand. But what if I have a gas cylinder at let's say 1atm, with an external pressure of 1 atm, at equilibrium. No weights on it. Let's say I pull the piston, I grab it with my hand and pull it. What will happen? At first I thought, ok, it will just do expansion work, reversible if I do it slowly, irreversible if I do it quickly... But then I thought, no that isn't possible, the gas is not doing any work, it's not like with the weights where the only upward force is caused by the gas molecules, now I'm actually pulling the piston. But my book says that if a gas expands against any moveable surface it's doing work... Could somebody help me?