Talk:Pressure

message: : Stagnation pressure' is the pressure a fluid exerts when it is motionless... although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure
Talking about the stagnation pressure of a moving fluid is very confusing when we've just defined that as the pressure it exerts when it is motionless - is it possible to clarify the definition a bit here? --Calair 22:49, 12 Mar 2005 (UTC)
thread_title: Stagnation pressure

message: A tow truck can exert a vast force in pulling a car without causing damage. However, a baseball bat directed against a certain part of car is likely to damage the car. That is because the focus of the bat exerts more pressure on that specific part of the car.
I don't think this is a good example. For starters, the wording implies (although doesn't actually state) that the towing force is greater than that inflicted by the bat. Some very rough calculations:
Baseball bat: ~ 1 kg.
Speed of swung bat: ~ 50 mph ~ 20 m/s (using figures for an untrained 12yo with a smaller bat )
Stopping distance: say, 0.02m (~1-inch dent.)
Assuming force during impact is proportional to indentation (i.e. y = -ky, y being indentation distance): from initial impact to stopping, y = a sin(bt) for some a & b, t being time from initial impact.
At stopping point, y' = 0, so a = 0.02m.
y' = ab cos(bt) = 0.02m*b*cos(bt)
At t=0, y'=20 m/s & cos(bt)=1
so b = (20 m/s)/0.02m = 1000/s.
y=-0.02m*b2*sin(bt)
At stopping point, sin(bt)=1 therefore y=-0.02m*(1000/s)2 = -20000 m/s2
so peak impact force ~ 20000 m/s2 * 1 kg = 20000 N.
Mass of big car: ~ 3000 kg.
Jeep Grand Cherokee, towing ~3000 kg load: 0 to 30 mph (~ 13 m/s) in 6.4 sec  (I couldn't find stats for tow truck acceleration, but I doubt they're much faster.)
so mean acceleration ~ 2 m/s2
so mean towing force during acceleration ~ 6000 N. (Peak forces will be somewhat higher, since acceleration isn't constant throughout takeoff.)
Getting past that, this is still comparing apples to oranges. If you're trying to dent somebody's car with a baseball bat, you're going for panels and the like; the towbar is rather more solidly built. Further, denting a panel is about bending, while towing is mostly straight compression/tension. If we want to show people the effects of a difference in pressure, we really need to apply both pressures to the same target in the same sort of way. 
The classic examples I always heard were "elephant standing on floor vs. person in stiletto heels on same floor" and "thumb pressing on corkboard vs point of thumbtack pressing on corkboard" - I think either of these would be preferable. --Calair 01:57, 13 Apr 2005 (UTC)
thread_title: Problems with example

message: The unit of pressure WC needs to be added to the list of units. This is commonly used to measure natural gas line pressure.
27.68" water column = 1 PSI
LexieM 21:53, 30 January 2006 (UTC)
replies: text: 'Inches of water' are listed in the Units section (under 'manometric', near the end). I think this is the same thing, but I've noted the abbreviation in the discussion of manometric pressure. Apologies for the multiple edits, kept getting things wrong today. --Calair 23:11, 30 January 2006 (UTC)
thread_title: Unit of Pressure: WC and ksca

message: As per discussion at gee, the difference in centripetal effects between equator & poles causes a variation of about 0.034m/s2 in perceived gravity over the Earth's surface at sea level; the effects of the equatorial bulge bring this up to about 0.052 m/s^2, so we should not be offering two decimal places of precision. When one adds in non-sea-level terrain, even "9.8 m/s2" starts to look like excessive precision. --Calair 22:57, 31 January 2006 (UTC)
replies: text: It is also 9.8 m/s² atop Mount Chimborazo, the highest mountain on Earth in both ways relevant here, and 9.8 m/s² at the Dead Sea, so your "at sea level" is exactly the same kind of excessive precision, just in words rather than in numbers. Gene Nygaard 02:50, 1 February 2006 (UTC)
replies: text: I agree with your revert, based on those numbers, but stipulating "at sea level" most certainly is not 'excessive precision'. In fact, it's the opposite of excessive precision - it's an unnecessarily weak statement (but a true one everywhere in the world), whereas the "9.81 m/s²" statement was an unjustifiably strong one that is false in some parts of the world. --Calair 03:47, 1 February 2006 (UTC)
thread_title: Appropriate precision for g

message: asap ..
The only thing to manage is to lie on the bed such that the pressure on all the nails is equal. In each and every surface there are some irregularities that interlock , thus a type of surface is formed by the nails.--115.240.129.38 (talk) 16:21, 6 September 2009 (UTC)
thread_title:  5.	Why is it possible to lie on a bed of nails and not be pierced? 

message: Rather than Nick Y's ill-informed rampant speculation about some "error" caused by "spell-checkers" in the last "10-15 years", let's just look at what the experts in some reliable sources have to say about that.  Here's Webster's Third New International Dictionary, Unabridges, 1961:
:¹gage \'gāj\ n -S [ME, from MF, of Gmc origin; akin to Goth wadī pledge —more at WED] 1 : a persional pledge that one will appear to support by combat his assertions or claims, esp : a glove, cap, or other belonging cast on the ground to be taken up by an opponent as a pledge of combat  2 : something deposited or given to or taken by another as a security for the performance of ome act by the person depositing it or giving it up and forfeited by nonperformance : SECURITY; also :the transaction by which the security is given or taken — compare MORTGAGE, PLEDGE — throw down the gage : CHALLENGE, DEFY
:²gage \"\ vt -ED/-ING/-S [MF gager, fr. OF gagler, fr. gage, n.] 1 archaic : tro give or deposit as a gage : give as security for some act : offer as a forfit : PLEDGE 2 archaic : STAKE, RISK
:³gage var of GAUGE
:4gage n -S \'gāj\ [by shortening] : GREENGAGE
Note that for definition 3, the use of this variant is not limited to a particular definition of the word.  Nor is it limited to a particular part of speech.  This variant is one used for any of the various meanings of gauge, whether used as a noun, as a verb, as an adjctive, or whatever. 
And, by the way, for the math-challenged among you, 1961 is more than "10-15 years ago."  Gene Nygaard 22:19, 12 December 2006 (UTC)
thread_title: References, Part I "gage"

message: Webster's Third, 1961:
:gauge pressure n : the pressure at a point in a fluid above that of the atmosphere — compare ABSOLUTE PRESSURE
Note that there is no entry for "gage pressure".  Gene Nygaard 22:43, 12 December 2006 (UTC)
thread_title: References, Part II "gage pressure"

message: All that really matters is the etymology.  Webster's Third, 1961:
:¹gauge or gage \gāj\ n -S [ME gauge, fr. ONF, prob. of Gmc origin; akin to OHG galgo cross, gallows — more at GALLOWS
You might want to go check out Middle English, to see if this likely results from the "use of spell-checkers in the past 10-15 years"!  It turned into Early Modern English over 600 years ago. What do you have to say now, Nick Y? Gene Nygaard 22:49, 12 December 2006 (UTC)
thread_title: References, Part III "gauge"

message: It seems "gauge pressure" is the consensus here --- Safemariner 04:57, 8 January 2007 (UTC)
thread_title: RFC response

message: [sigh...] Where to begin? I am a humble copy editor who came across this article because it was tagged with . The issue was an inconsistency in the spelling of the word gauge (also spelled gage)—or is it gage (also spelled gauge)? Words cannot express my reluctance to revisit this debate, which I thought was resolved in January 2007. Here is what I will do:
# I will honor the outcome of the RfC ("It seems ‘gauge pressure’ is the consensus here --- Safemariner 04:57, 8 January 2007 [UTC]") and change all instances of the word gage to gauge.
# At the first mention of the word gauge, I will include a parenthetical clause which will give equal weight to the alternate spelling: gage.
# I will attach a footnote, which was already written, to the first (and only) mention of gage.
I hope this will address the copy edit issue, and respect the consensus of the editors involved. Cheers. Braincricket (talk) 09:22, 4 January 2012 (UTC)
thread_title: January 2012 copy edit tag

message: To unnecessarily (and probably unwisely) re-visit a bitter (and long...I ran out of popcorn!) debate, I just felt I had to add my own personal observation that it always strikes me as foolish when people act as if a change in a words pronunciation or spelling was desecrating thousands of years of history written in stone, when language has changed constantly throughout history (and will continue to do so), and when "official spellings" as such were non-existent or unknown up until quite recently. At some point, people decided they would sit down and freeze language into stone, which I think is a futile attempt doomed to failure, and whenever certain people see other people using the "official" language "incorrectly", they form a mob and break out the torches. It's even worse in cases like this where they disagree on what the official spelling even is. Granted, in fields of scientific endeavor, some terms must be made constant and uniform, but in this particular case, does it really matter that much whether it's spelled "gage" or "gauge"? You might loose a little precision, but the while thing isn't worth starting a jihad over, especially since it's unlikely that proponents of one variant will ever change the way people in another country spell the word. I think it's wiser to simply educate people if they see "gauge pressure" or "gage pressure", they may well mean the same thing, and it's up to them to be aware of this and use common sense. Most people dealing with this sort of thing ought to be intelligent enough to muddle through a small difference in spelling if they know that it could mean either thing. Personally, I prefer "gauge", for the simple reason that I consider "gage" to be ugly (a dumb reason, yes). That's also why I use "grey" instead of "gray", even though it's "incorrect" to do so. Unless you live in the UK, anyway.
One question though, I'd genuinely like to know...one sentence in the above debate seems to be saying that "gage" is pronounced differently than "gauge". Is this true? He says that 
    "You can walk into any automotive supply store and if you ask what the "g" in psig means, some "Mr. Goodwrench"-looking manager will verbally say it stands for "gauge". The average fellow will assume the word is spelled like "pressure gauge." It's a  phenomenon much like the pronunciation of "giga" (as in gigawatts or gigabytes). Originally (before the 1990s), it took its pronunciation from its root meaning and sounded like "gigantic." Once the average Joe read it over and over in computer magazines but never heard the word in use, they assume it's pronounced with a hard "G."
So "gage" is pronounced "jahy-juh" then? That's certainly a education for me. If it's not true, I fail to see what the relevance is...is the auto store man supposed to spell it out for him? How are two words spelled differently and pronounced the same the same thing as a word that was simply misread and entered public consciousness in a way it wasn't originally intended to do. A "gigawatt" is a "gigawatt", regardless of how you pronounce it, while the whole point of the complaining side of this debate is that that "gauge" and "gage" have two different meanings, in spite of being pronounced the same. I fail to see any real parallel there.
As a last note, for what it's worth, I was taught at some place or other that "gage" is simply an American simplification of the older British term "gauge", but both are considered correct spellings in the US (of course, in that context it was referring to "gauges" such as instruments). I believe I looked it up, because the first time I saw "gage" written out, I assumed it was a spelling error, because I'd never seen it spelled that way before..45Colt 07:48, 19 August 2015 (UTC)
thread_title: Hello

message: I deleted the Biology part since it was just vandalism by someone who was trying to be funny.
Ranmamaru 06:29, 11 January 2007 (UTC)
thread_title:  deleted BIOLOGY 

message: ok so volume of a liquid has to be conserved (given it is incompressible, non viscous blah blah) but why should the force per unit area be conserved and be the same in all directions?
Basically if I just model everything as volume conservation and simply try and say that the liquid "en mass" will try to attain a level of minimal potential energy, I still end up with the same results. However to make the assumption that dF/dA is conserved seems off beat to me. —The preceding unsigned comment was added by 220.227.207.194 (talk) 09:28, 5 March 2007 (UTC)
thread_title:   Pascal's law concept beats me 

message: My thanks to the author that wrote the description of pressure in this section, where it is written that if you add some new surface within some gas container, that the pressure is going to still be the same no matter where the new surface is placed (well, also assuming that you make adjustments with the original surfaces that you are going to be comparing to). Now, what I would like to know is what the actual "force (mass-acceleration) per unit area" actually is. Clearly, pressure is going to be defined as the gas molecules that are bumping into the surface making up its container, but what does that mean? Does that mean that the size of the molecule is making some sort of "contact plane" with the side of the container? What about when the container of the gas is spherical, there'd be finite size to the molecules but not to the container (assume it is perfectly spherical, so there's no definite points of contacts, only intervals, right?), what then? That "contact plane" is what I am mostly asking about. The origin of this all is my own questioning on what is pressure, to better understand Avogadro's law. I know that these are approximations and that one could simply deal with the given variables, but I am more interested in the complexities that arise in the bigger questions. Thank you, -- kanzure 17:52, 22 March 2007 (UTC)
thread_title:  Re: Scalar nature of pressure 

message: I don't have the experience to fix what is wrong with this page... The math functions are all messed up.
:How so?  --Selket Talk 23:04, 17 May 2007 (UTC)
replies: text: The helpme template has been removed for now. Please put it back when you answer, so we know to check back. Thanks--Werdan7T @ 00:00, 18 May 2007 (UTC)
thread_title:  fix the functions 

message: (Trying to settle a small revert war between me and another user.)
People from Commonwealth countries often refer to U.S. units as "Imperial Units". That is not correct. U.S. units are a simplification of the customary systems that the colonies inherited from England. These were not Imperial units, which were not defined until 1824, long after the U.S. became independent.
Some units are the same in both systems, including weight. Others have the same name but are not the same. An Imperial gallon, for example, is larger than a U.S. gallon.
Thus it does not make sense to refer to any measure based on the customary definition of the pound as "US" (the Imperial system uses it too) or "Imperial" (the unit is used in at least one other system). --Isaac R (talk) 23:03, 26 July 2008 (UTC)
replies: text: Uh, not a revert war.  Customary units is a vague term and I'm not sure a proper one.  This article only lists the unit names, so standards is not relevant.  For maodern usage they should be labeled US Customary units.  -Fnlayson (talk) 00:04, 27 July 2008 (UTC)
replies: text:  A respectful revert conflict? ¶ You're right, "customary units" is vague, probably not the right term. But I don't know what the right term is. I just know that "Imperial" and "U.S." are both wrong and misleading, since neither system owns the International pound. (See pound (mass).) Perhaps "International" is the correct word, but I'm not sure that adjective applies to units derived from the International pound. Isaac R (talk) 00:40, 27 July 2008 (UTC)
replies: text:  It'll be OK as "customary units" unless someone has a better, more correct name. -Fnlayson (talk) 00:49, 27 July 2008 (UTC)
replies: text:  Consider English units.Headbomb {ταλκ – WP Physics: PotW} 05:21, 27 July 2008 (UTC)
replies: text:  "English units" refers to the 18th century hodgepodge that U.S. units are based on. Which did not include a unit of pressure. ¶ Blatant editorialization: the fact that we're even having this discussion should be profoundly embarrassing to every American who works in any scientific or technical field. Our inability to make the change to the metric system is a symptom of our cultural arrogance and shortsightedness. Isaac R (talk) 17:05, 18 August 2008 (UTC)
replies: text: If it's the units of mass we're talking about, we can call them avoirdupois, otherwise how about imperial/US units?  We certainly wouldn't call them international this would seem to imply SI units. JIMp talk·cont 20:21, 7 July 2010 (UTC)
thread_title:  What are "Imperial Units"? 

message: I find these tables that compare various units interesting but
1) they are also confusing to the novice because they do not explicitly explain how to convert
2) they are necessarily limited to about a half dozen columns, that is, only relations among a few units can be shown.
Instead I suggest basing a new table on the standard international unit with all the other units listed with their explicit conversion factors to and from the SIU.  Joseph Grcar (talk) 16:18, 29 July 2008 (UTC)
thread_title:  About the table 

message: Given that a real gas has an unknown fundamental thermodynamic relation, its equation of state must necessarily also be unknown, therefore I will remove the reference to an equation for a real gas. LeBofSportif (talk) 22:39, 29 July 2008 (UTC)
thread_title:  Pressure of real gas 

message: * The order of magnitude seems out by a factor of 1000 in the relationship between Pa and atm. Nesbit (talk) 18:53, 19 September 2008 (UTC)
thread_title:  Pa:atm conversion in table is weird 

message: Shouldn't a statistical definition to be included on this page be proper?  —Preceding unsigned comment added by Mppf (talk • contribs) 01:07, 11 August 2010 (UTC)
thread_title:  Statistical definition 

message: 1 Pa should equal 0.00001 Bar, not 0.0001 Bar.  — Preceding unsigned comment added by 166.147.64.27 (talk) 16:39, 11 September 2011 (UTC)
thread_title:  Mistake in chart? 

message: A well must be perforated at 8500 ft.  The casing is full of 9.5 ppg mud.  The well has 32 degree API gravity oil in the well and the fluid level is at 4400 ft. with a gas surface pressure of 820 psi and a gas gravity of .8.  What is the pressure differential between the casing and tubing? How do I figure this out??
                                                                        32 degree API gravity oil is = 7.22 lbs/gal    
                                                                         the gas gravity of .8 is = 1.2894  — Preceding unsigned comment added by 12.230.15.34 (talk) 16:44, 14 November 2012 (UTC)
thread_title:  Question 

message: I expected to find a mention of Pascal's vases in this article, but there isn't one; nor do they seem to have an article of their own. Are Pascal's vases just considered to be a historical curiosity and not important any more? Dezaxa (talk) 13:52, 4 January 2013 (UTC)
thread_title:  Pascal's Vases 

message: I would really contest the phrase "Negative pressure must exist at the top of any tree taller than 10 m". It doesn't sound scientific at all... You imagine a 50 meters tall ladder with minions that pass a bucket of water from hand to hand to the top... There is no negative pressure on the ladder, even if the minions are soaked wet and touch each other... In fact, the whole ascent of sap article (linked at the negative pressure section) is kinda gibberish...(if it would be true, then the sap will raise to the top of a dead tree too, because there is no structural difference between green wood and dead wood, they have the same capillarity, etc).  — Preceding unsigned comment added by 223.27.210.130 (talk) 10:53, 25 October 2013 (UTC)
replies: text: I agree. I have never been convinced by the arguments put forward in favor of negative pressures existing in fluids. Dolphin (t) 11:54, 25 October 2013 (UTC)
replies: text: I think what is meant is an apparent negative pressure acting on water molecules only. A real negative pressure would imply a vacuum that whooshes everything in - air molecules, leaves outside the cell, etc., which is nonsense. However intermolecular forces are specific to the molecules interacting, and there are attractive forces on the water molecules which specifically pull water molecules up the tall tree. The article Ascent of sap in fact refers to the apparent existence of large negative pressures in some living plants. I will change this article to express this more accurately. Dirac66 (talk) 22:32, 6 August 2014 (UTC)
replies: text:  Note that the negative pressure acts on the wall of the capillary as well, which has to be quite strong or it would implode. Petr Matas 15:37, 7 August 2014 (UTC)
replies: text: It is not nonsense. I've cited the existence of negative pressures in the article now. Oreo Priest talk 13:24, 31 January 2015 (UTC)
thread_title:  Negative pressure 

message: The introductory section of this article contains the following sentence:
 A pressure of 1 Pa is small; it approximately equals the pressure exerted by a dollar bill resting flat on a table.
This sentence is parochial: it would be an unhelpful example to anyone not familiar with US currency. Such an example is unhelpful. I suggest removing it unless a replacement can be found which is less culturally specific.
Chrislaing (talk) 21:20, 9 December 2013 (UTC)
replies: text: I agree. We should just get rid of it.  This is about pressure not the pascal anyway. Jimp 15:36, 24 February 2015 (UTC)
thread_title:  Introductory example of a "dollar bill" is parochial 

message: Hello , I think that in lists and comparisons of pressure units, each unit should wikilink to the article about the unit, even though it has been wikilinked already. I think that at such places the wikilink usefulness outweighs the repeated link ugliness. Petr Matas 20:11, 17 August 2014 (UTC)
thread_title:  Wikilinks to units 

message: I think that 's statement about energy equation is incorrect. Unlike pressure, the gravitational potential energy is not a function of position of measurement, but rather of position of the liquid body's center of gravity (and depends also on the choice of the zero height level), because it is an integral quantity. Therefore, the statement "energy is constant throughout the vessel" is a nonsense. Unless this is fixed somehow (I doubt that it is possible), I think that the paragraph should be removed. Petr Matas 06:46, 7 November 2014 (UTC)
replies: text: * ??.  It's more clear with unit volume but the concept is the same.  A cubic-centimeter at the top of a fluid has the same total energy as a CC at the bottom.  The top surface CC has more gravitational potential energy and less pressure energy while the CC at the bottom has more pressure energy and less gravity energy.  The sum is constant.  (gravity head + pressure head = constant)  --DHeyward (talk) 22:43, 7 November 2014 (UTC)
replies: text: ** I was confused by the expression "total energy" - I thought that you were speaking about the energy of the entire liquid body. The body's position is given, so the expression "throughout the body" is meaningless for it. When it's per volume element, it's ok. Petr Matas 16:12, 8 November 2014 (UTC)
replies: text: * Ahh.  It's a property of fluids.  The center of mass potential energy is give but the tradeoff as the energy is calculated away from the center is the same at any other point.  Per unit volume is easier to express and understand.  The principle is the same as the volume can be made infinitely small such that a point calculation yields the same constant.  It's the reason a deep ocean of 2 miles depth has the same level as the same ocean in two feet of water.  Take an empty tube that's two miles long with a valve at the bottom, submerse it to 2 miles and open the valve - the water rises to the same level as the rest of the ocean because the energy is constant.  --DHeyward (talk) 04:42, 9 November 2014 (UTC)

text: *:@DHeyward how??????? 41.210.141.245 (talk) 11:15, 13 May 2024 (UTC)
thread_title:  Liquid pressure as energy equation 

message: I was surprised that I can't seem to find an article or section anywhere on Wikipedia where it actually discusses compression heating and decompression cooling, except for a brief bit in Compression ignition engine. Probably there is some discussion in pages on HVAC and refrigeration as well, but what I'm looking for is a technical page I can link to so people can read about why fluids (i.e. air) heat up when pressurized (well, compressed, anyway). I've looked on the page on Compressed air, and it says nothing. I believe it mentioned it on the page on Bleed air as well (it's been a while), but the point is, that none of these are really relevant to link to if I wanted to link to a page explaining why air heats as it's compressed. For example, I was on the page on Gas compressor(or maybe it was the page on Rotary screw compressors), and it mentions the elements of the system dedicated to "cooling the compressed air", but it doesn't say anything else. I thought it might be useful to link to a page explaining why the air would NEED cooling, but I can't find anything, unless I wanted to link to the page on diesel engines and let them read between the lines. Which doesn't seem very encyclopedic, somehow..45Colt 07:16, 19 August 2015 (UTC)
replies: text: You could link to Adiabatic process#Adiabatic heating and cooling. NebY (talk) 20:00, 19 August 2015 (UTC)
thread_title:  Pressure as relates to heating/cooling or fluids 

message: Pressure on ears is the same as pressure on other parts of the body at the same depth. Why specify ears? • • • Peter (Southwood) (talk): 05:59, 5 August 2016 (UTC)
thread_title:  Pressure on ears? 

message: Is there a variant of pressure that is used for force over 3-dimensional volume?? If not, please explain why. Georgia guy (talk) 14:59, 1 May 2017 (UTC)
thread_title:  Force over volume?? 

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thread_title:  External links modified