I'm feeling pretty silly not being able to understand this. Let's say that you have a piston (red) sitting inside of a housing (black) like this Let's say that something like this was lying on its side on a flat table. By my understanding, if the contact between the piston and the housing is frictionless and all of the air is sucked out of the central cavity, then the only forces acting on the red piston come from the atmospheric pressure acting on the two surfaces. Since atmospheric pressure acts on a greater surface area on the left compared to the right, it would seem that there would be a greater force acting on the left than on the right, and the piston would slide to the right. This result seems completely inane, but I can't for the life of me justify the expectation of nothing moving using a statics force balance. Could someone help me out with this?