Fluidsare materials that can flow. The mass densityρof any substance is its massm divided by its volume V: ρ= m/V.In the presence of gravity, the upper layers of a fluidpush downward on the layers beneath, with the result that fluid pressure is related to depth.In an incompressible static fluid whose density is ρ, the relation is P2= P1+ ρgh, where P1is the pressure at one level and P2is the pressure at a level that is hmeters deeper. The gauge pressureis the amount by which a pressure Pdiffers from atmospheric pressure. The absolute pressureis the actual value for P.According to Pascal's principle,any change in the pressureapplied to a completely enclosed fluidis transmitted undiminished to all parts of the fluid and the enclosing walls.The buoyant forceis the net upward forcethat a fluidapplies to any object that is immersed partially or completely in it. Archimedes' principlestates that the magnitude of the buoyant force equals the weightof the fluid that the immersed object displaces.
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