The Classical Piston and CylinderThe geometry of a piston (with mass) that snuggly fits circular piston (no leakage) and which moves freely therein (without friction) is a false but highly used premise of physical reality widely used by thermodynamics. The frictionless piston/cylinder is a hypothetical contrivances of classical thermodynamics used to hypothecate a constant pressure environment for some system and its event. To detemine the system pressure apply the momentum equation to the piston.

The above sketch shows three allignments for the classical piston/cylinder device.