Pressure and Buoyancy



PB – PA = Dgh

Where g = 10m/s2 on the earth. If the liquid is water, D = 1,000kg/m3, and the height difference is 2m, the pressure at B is 20,000N/m2, or 20,000Pa, greater than the pressure at A.

One way of understanding this is to note that the fluid at B must support the weight of the material above it, and there is more material above B than above A.




This yields the simple result that if an object is submerged in a liquid or gas and is more dense than the surrounding liquid or gas, it will weigh more than the material it displaces and sink.  If it is less dense, it will weigh less than the material it displaces and rise.