# Is Air A Fluid? **URL:** \ **Category:** Factual Questions\ **Created:** [November 9, 2002, 12:27pm UTC](https://boards.straightdope.com/t/is-air-a-fluid/136016 "2002-11-09T12:27:25Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![bobk2](https://avatars.discourse-cdn.com/v4/letter/b/6de8d8/32.png) [@bobk2](https://boards.straightdope.com/u/bobk2)\ **Post date:** [November 11, 2002, 5:36pm UTC](https://boards.straightdope.com/t/is-air-a-fluid/136016/21 "2002-11-11T17:36:36Z")
Pardon if this was addressed previously, but in High School Chemistry and Physics, didn’t they say that there were three states ; solid, liquid, gas. Does this thread mean there’s really only 2 states; solid and fluid? ( I realize the 3 states don’t apply to everything ; I can’t get wood to melt. And I’m not sure how ‘plasma’ is categorized. ) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 11, 2002, 6:45pm UTC](https://boards.straightdope.com/t/is-air-a-fluid/136016/22 "2002-11-11T18:45:46Z")
“Fluid” isn’t a single state. Under normal terrestrial conditions, it includes two states: Solid and liquid. Plasma is also a fluid, but seldom encountered on Earth. And a given substance can also have several different solid states, with different crystal structures. Just because wood doesn’t melt, by the way, doesn’t mean that the simple solid-liquid-gas model breaks down for it. It doesn’t have a recognizable liquid or gas phase (i.e., before wood melts, it’ll turn into something other than wood), but you can still perfectly well call wood a “solid”. If you want to really stretch your brain, under some circumstances, there’s no clear distinction between liquids and gasses: You can go from, say, liquid water to gaseous water without ever actually going through a phase transition. The way to think of it is this: A liquid is a fluid with the equation of state density = constant. A gas is a fluid with the equation of state density = pressure/(temperature\*Boltzman’s constant). It’s quite possible to have an equation of state somewhere between those two, allowing for a gradual transition. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [November 11, 2002, 7:16pm UTC](https://boards.straightdope.com/t/is-air-a-fluid/136016/23 "2002-11-11T19:16:10Z")
_“Fluid”…includes two states: Solid and liquid._ I’m sure he meant to say _gas and liquid_. ---
**Author:** ![honkytonkwillie](https://avatars.discourse-cdn.com/v4/letter/h/3da27b/32.png) [@honkytonkwillie](https://boards.straightdope.com/u/honkytonkwillie)\ **Post date:** [November 12, 2002, 8:00am UTC](https://boards.straightdope.com/t/is-air-a-fluid/136016/24 "2002-11-12T08:00:40Z")
> [@](#): > > \*Originally posted by David Simmons \* > \*\*This is the venturi effect. \*\* Ah. I get it. Thanks. ---
**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\ **Post date:** [November 12, 2002, 8:59am UTC](https://boards.straightdope.com/t/is-air-a-fluid/136016/25 "2002-11-12T08:59:31Z")
Water is a fluid, and so is air. An aircraft would fly just fine underwater, and in fact the same math applies to sailboat keels as applies to sailboat sails. Water is more massive than air of course. And air is compressible, but this has no impact except when airfoils get near the speed of sound. > [@](#): > > However, Newton’s action/reaction laws don’t explain how certain airfoils at zero angle of attack can still generate lift. Yes they do. Certain airfoils (cambered airfoils) generate lift at zero angle of attack because they deflect air downwards. They deflect air downwards because the trailing edge of the airfoil has a far greater effect on the air than the leading edge does. It’s a consequence of inertia. In other words, it doesn’t matter if the entire airfoil is at zero AOA, if the trailing edge aims downwards, then the wing generates a lifting force. This is called “Kutta Condition” in intro aerodynamics. A flat-bottomed curved-top wing has a trailing edge which aims downwards. Here’s a book by a couple of aerodynamics researchers which debunks extremely widespread misconceptions about wings: Understanding Flight [http://www.amazon.com/exec/obidos/ASIN/0071363777/qid%3D1012330664/](http://www.amazon.com/exec/obidos/ASIN/0071363777/qid%3D1012330664/) And an article by the same authors: A physical description of flight [http://www.aa.washington.edu/faculty/eberhardt/lift.htm](http://www.aa.washington.edu/faculty/eberhardt/lift.htm) Here’s a good website from a pilots’ training course: [http://www.monmouth.com/~jsd/fly/how/htm/airfoils.html](http://www.monmouth.com/~jsd/fly/how/htm/airfoils.html) > [@](#): > > Or how a strong enough wind can pull the roof or door off a building. Bernoulli’s equation is entirely based on Newton’s laws. You can ignore Bernoulli entirely and just apply Newton to moving parcels of air, and you’ll get the same results. I hear that’s how the aerodynamics simulation software does it. PS, Bernoulli only applies to one-dimensional or straight line flow, and it doesn’t describe the air pressure caused by flow-deflection (by streamline curvature.) Also: my website on misconceptions which infest grade-school books about airplanes (and until recently were also in pilot-training textbooks, and even in pilot liscencing exams!) K-6 lifting force misconceptions [http://amasci.com/wing/airfoil.html](http://amasci.com/wing/airfoil.html) ---
**Author:** ![Cartooniverse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cartooniverse/32/3084_2.png) [@Cartooniverse](https://boards.straightdope.com/u/Cartooniverse)\ **Post date:** [November 12, 2002, 10:51pm UTC](https://boards.straightdope.com/t/is-air-a-fluid/136016/26 "2002-11-12T22:51:19Z")
…god, this is cool. My thanks to those who are truly in the know here. Keep going ! [Previous page](https://boards.straightdope.com/t/is-air-a-fluid/136016.md?page=1)