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? http://img235.imageshack.us/img235/4540/piston1lh2.th.jpg [Broken] 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.