This is problem one on my worksheet, and all the rest I've found to be easier, so I'm thinking I'm missing something here: A vertical cylinder fitted with a frictionless piston contains air at 100C The piston has an unknown m and a diameter of .150mm, and the ambient pressure outside the cylinder is 101kPa. If the cylinder volume is 500 liters and the mass of the air is 5kg, what is the piston mass, m? I calculated the pressure inside the cylinder and it is huge. That means the piston would accelerate upwards. Since the sum of the forces on the piston must equal its mass times acceleration, I used something like (AFTER i had obtained the inner pressure): p(sys)*a(piston) - p(ambient)*a(piston) - mass(piston)*gravity = mass(piston)*acceleration(piston) The trouble is, is that i know neither the acceleration or the mass of the piston. THe only way this can work is if I assume that the piston is not moving, but I don't think I can just assume that?