1. The problem statement, all variables and given/known data The question is: A 2 litre cooking pot (diameter: 20 cm, mass: 300g) is closed with a tight fitting lid, and contains only air. What mass of air can be maximally in the pot so that you can lift the lid, and the pot will move with the lid? Assume the surrounding pressure is p0 = 0.9 bar, and the temperature is 19°C. 2. Relevant equations pV=mRT F = Fp + FG 3. The attempt at a solution I'm stuck on the free body diagram for this question. I'm thinking that if the pressure inside the pot is lower than the outside, the lid will stay on as the pot is lifted (obviously). I think this might be the force equation: F = Fp + FG where F = force upwards on lid, Fp = force of lower air pressure inside pulling down on lid, and FG = force down on (mass of pot + mass of air) by gravity. First of all, I'm not sure if that makes any sense. Does it take extra force upwards to compensate for the force pulling down on the lid due to negative air pressure/force pushing down on the lid by the greater outside pressure? Also, how can I figure out any of the forces if the mass of air is unknown? I just don't know where to start and I can find no relevant examples that explain this.