1. The problem statement, all variables and given/known data A cylinder with a frictionless piston of mass M and cross section S is placed vertically in an atmosphere of pressure p. The cylinder is rotated 180 degree so that the opening of the cylinder faces down. During the operation the temperature of the gas inside the cylinder is kept constant and the volume of the inside the gas is doubled. The acceleration due to the gravity is denoted by g. 2. Relevant equations Which of these are correct? a) p=3Mg/S b) p=2Mg/S c) p=Mg/S d) p=Mg/2S e) p=Mg/3S 3. The attempt at a solution I don't have a clue.