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Physics - Hydrostatic Pressure

Fluid pressure is the pressure that would be exerted on an object if it was submerged in a fluid. Fluids have fluid pressure even if nothing is submerged in them. At any given depth, a fluid’s fluid pressure is equal to the pressure that it would exert on a submerged object. 

Rule number 21: 
If fluid is inside a closed container and is subject to no forces other than gravity, then the fluid pressure at any point in the fluid is equal to the fluid’s density times G times the height of the fluid sitting above the point. This rule applies regardless of the shape of the container in which the fluid is located. The reason for this is Pascal’s Law, which states that when pressure is increased at any point in a contained fluid, the increase is uniformly distributed throughout the fluid. Now, what about fluid-filled containers that are open to the atmosphere? 

Rule number 22: 
Total pressure in the fluid equals gauge pressure plus atmospheric pressure. Do not forget that gases are also fluids, and the laws that apply to fluids apply to gases too. Fluids produce buoyancy. 

Rule number 23: 
Any object that is partly or wholly immersed in any fluid experiences a buoyant force equal to the weight of the fluid it displaces. Buoyant force equals volume of displaced fluid times density of displaced fluid times gravity. This is known as Archimedes’ principle.