Im having a spot of trouble with some pretty elementary problem. A mass sits inside a tube which is pointed vertically, the mass is the same shape as the pipe ( i.e. it behaves like a piston ), but the piston diameter is less than that of the pipe. In order to lift the piston, what pressure must be applied to the fluid? The top of the chamber is eventually open to the atmosphere, and is filled with air at STP. My problem seems to be the lack of a seal about the piston, i can see that it would be simple to calculate if it was simplified to a sealed piston, but with a piston thats smaller than the bore of the chamber its harder.