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Why does a faster air flow cause a lower pressure?

The full statement of the principle is that, along a given airstream (following the flow of the air) that where the air is moving faster the pressure is lower, and where the air is moving slower the pressure is higher. Since the total quantity of air moving along with the flow per unit time has to be the same everywhere, there must be less of it (per unit distance) in the faster region, and hence a lower pressure. At least that is the explanation I remember being given a very long time ago - now that I think more about it there seems to be something wrong with that explanation because the usual equations for fluid mechanics assume a constant density (well maybe that is because air is not the same as water?) Anyway, good question, and maybe somebody who knows more about it can give a better answer?

Arthur Smith

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