I was taught that pressure difference in a fluid is the cause of buoyancy force, but I when I think of the buoyancy force at the bottom of a fluid, this explanation seems to give me some weird results. Suppose a have a cylindrical block placed in the middle of the fluid. Then the cause of buoyancy force is clear, since pressure exert normal to the surface, the curved surface's force add up to be 0. Only the upper part and the lower part of the cylinder contribute to the force, and by simple calculations, we have the force = weight of displaced fluid. However, suppose I keep lowering the cylinder, just until it touches the ground. Since pressure is exert by the fluid, and the bottom of the cylinder is NOT touching any fluid. It should have no pressure, and the only pressure is on the top of the cylinder. Pushing the cylinder downward, the buoyancy force turned downward! However, we know that it should not be the case. So where is the problem ? Where is thr fault of the above problem? I really confused me and any help would be appreciated
No,there will be no buoyant force on the cylinder. Actually here the cylinder,apart from its weight and reaction,will experience a downward force F=(p+hdg)A due to hydrostatic pressure where p is atmospheric pressure,h is depth of upper surface of cylinder,d is density of liquid,g is acceleration due to gravity and A is area of plane surface of the cylinder.