1. The problem statement, all variables and given/known data A cylinder is vertically placed with its frictionless ,light and moveable piston facing the top. A mass of known value is placed on it, and its distance from the bottom of the cylinder is known. What is the pressure of the gas ? 2. Relevant equations 3. The attempt at a solution The potential energy of the mass is the work done by the gas. Since the system is in equilibrium , the gas must do work against the compressing effect of the piston.But the problem is there is no change in the volume of the gas. W=p dV is simply zero since dV is 0 . That doesn't make sense ??