Some Rules about Hydrostatics (ME 33 Handout, prepared by Professor J. M. Cimbala) •For hydrostatics, pressure can be found from the simple equation, belowabovePPgzρ=+∆•There are several “rules” that directly result from the above equation: 1.If you can draw a continuous line through the samefluid from point 1to point 2, then P1= P2if z1= z2 .E.g., consider the oddly shaped container in the sketch. By this rule, P1 = P2and P4 = P5since these points are at the same elevation in the same fluid. However, P2does not equal P3even though they are at the same elevation, because one cannot draw a line connecting these points through the samefluid. In fact, P2is less thanP3since mercury is denser than water. 2.Any free surface open to the atmosphere has atmospheric pressure, Patm.(This rule holds not only for hydrostatics, by the way, but for anyfree surface exposed to the atmosphere, whether that surface is moving, stationary, flat, or curved.) Consider the hydrostatics example of a container of water. The little upside-down triangle indicates a free surface, and means
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