A piston is free to move in a cylinder which contains a gas. The gas exerts a force of 10N on the piston, and thus by Newton's 3rd law, the piston exerts a force of 10N on the gas. Hence the gas and piston move in opposite directions, which is contradictory. How do I properly apply Newton's 3rd law to a case like this?
Also, this is completely different from what the intended question, but does the expression $W = PV$ apply to liquids or solids expanding? Does it apply when solids are not expanding, but are being moved, for e.g a cube is physically moved 4 meters by a constant horizontal force of 10N?