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Some_rules_about_hydrostatics

Some_rules_about_hydrostatics - Some Rules about...

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Some Rules about Hydrostatics (ME 33 Handout, prepared by Professor J. M. Cimbala) For hydrostatics, pressure can be found from the simple equation, below above P P g z ρ = + There are several “rules” that directly result from the above equation: 1. If you can draw a continuous line through the same fluid from point 1 to point 2 , then P 1 = P 2 if z 1 = z 2 . E.g., consider the oddly shaped container in the sketch. By this rule, P 1 = P 2 and P 4 = P 5 since these points are at the same elevation in the same fluid. However, P 2 does not equal P 3 even though they are at the same elevation, because one cannot draw a line connecting these points through the same fluid. In fact, P 2 is less than P 3 since mercury is denser than water. 2. Any free surface open to the atmosphere has atmospheric pressure, P atm . (This rule holds not only for hydrostatics, by the way, but for any free 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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