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Which is more appropriate regarding Bernoulli's principle

  • fast moving air causes low pressure or
  • lower pressure causes fast moving air.
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Low pressure in itself does not create any flow, as the pressure is the same everywhere. Bernoulli's principle should always be applied between two point along a streamline. When doing this, you will find that only the pressure difference between to points play a role.

Then, it comes down to the specific problem your are studying. Sometimes you will have information about velocities, sometimes about the pressure or the height. There is now general statement possible.

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  • $\begingroup$ Well if low pressure and high pressure exist in two adjacent regions, air will flow.I know 1 is correct.Can you tell if 2 is? $\endgroup$ – soumyadeep Oct 26 '13 at 10:52
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    $\begingroup$ Not per se: en.wikipedia.org/wiki/Adverse_pressure_gradient $\endgroup$ – Bernhard Oct 26 '13 at 11:15
  • $\begingroup$ What do you mean by"per se"?Anyway,i read the article.It would be great if you explain the second paragraph of the article.Moreover the article says if pressure is highered velocity will reduce.But if pressure is lowered in the direction air is flowing i.e.P.E is reduced by Bernoulli's principle the K.E and hence will increase.So 2 is correct,isn't it?? $\endgroup$ – soumyadeep Oct 26 '13 at 17:25
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    $\begingroup$ 2 makes no sense. as @Bernhard says, only pressure differences generate accelerations (forces). If the pressure is everywhere low, there is no pressure difference and hence it does not cause the air to move faster. $\endgroup$ – Isopycnal Oscillation Oct 27 '13 at 17:57
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Neither of your two statements have any validity to them and are therefore not applicable to understanding Bernoulli's principle, let alone more appropriate than the other. Pressure is a relative quantity that has little meaning by itself. Changes in pressure have meaning and can cause fluid motion. For situations where irrotational flow is a valid assumption, the primary driving force behind fluid motion is the need to respond to boundary surface geometry.

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  • $\begingroup$ This is the question of a physics book-come on. $\endgroup$ – soumyadeep Oct 29 '13 at 14:03
  • $\begingroup$ Physics books are never wrong? Is that what you are telling me? Perhaps you have removed some of the context of what is in the book? Care to share the name of the book? $\endgroup$ – SimpleLikeAnEgg Oct 29 '13 at 21:04
  • $\begingroup$ "university physics"-by young and freedman 13th edition pg389 $\endgroup$ – soumyadeep Oct 30 '13 at 10:07
  • $\begingroup$ And please-i do not like your tone $\endgroup$ – soumyadeep Oct 30 '13 at 10:08
  • $\begingroup$ When reading text books, particularly those that are generalized introductions to a wide variety of topics, you are best to approach everything with skepticism. This is a perfect example of a question posed by an author that is most likely not an expert in fluid mechanics. The two answers you have received are entirely accurate. Whether you like the tone or not should be irrelevant to you. The only thing that should matter to you is the truth. If you don't like the tone, then simply move on and ignore the answer, in this case to your own detriment. $\endgroup$ – SimpleLikeAnEgg Oct 30 '13 at 17:58
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Definitely the second, assuming that what you meant to say was "Relatively low pressure causes fast moving air." Bernhard is absolutely correct on this point, in that really it's the pressure gradient, not the pressure itself which causes local accelerations in the flow. The language I am using is very deliberate, suggesting that the pressure gradient causes the flow accelerations and not the other way round. After all, Newton's Second Law tells us what an object will do when a net force is applied to it, not what forces will be generated when it just begins to move apropos of nothing. Forces cause accelerations, and so analogously the pressure gradients are what cause the local flow accelerations (although the phenomena are dynamically coupled).

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protected by ACuriousMind Aug 20 '15 at 2:34

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