
Pressure is the application of force to a surface, and the concentration of that force in a given area. A finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a thumbtack can easily damage the wall, even though the force applied is the same, because the point concentrates that force into a smaller area. More formally, pressure (symbol: ''p'' or ''P'') is the measure of the surface normal component of force (physics) that acts on a unit area, see also stress (physics): : where: *''p'' is the pressure *''F'' is the normal component of the force *''A'' is the area Often ''F'' is taken to be the magnitude of the mean vector force normal to the surface of area ''A'' upon which it exerts; the "surface" not necessarily being a that of a body, but for example the cross sectional area of a conduit. The gradient of pressure is force density. Pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure; such measurements are sometimes called gauge pressure. An example of this is the air pressure in a tire of a car, which might be said to be "thirty PSI", but is actually thirty PSI above atmospheric pressure. In technical work, this is often written as "30 PSIG" or, more commonly, "30 psig", though other methods which avoid attaching this information to the unit of pressure are preferred. [http://physics.nist.gov/Pubs/SP811/sec07.html#7.4 1] In the human body, pressure is measured by baroreceptors. "Pressure is a scalar quantity, but teachers and authors do not appear to believe this in their hearts." (McClelland, 1987) ==Scalar quantity== Let us look at a static gas; one that does not appear to move or flow. While the gas as a whole does not appear to move, the individual molecules of the gas, which we cannot see, are in constant random motion. Because we are dealing with a nearly infinite number of molecules and because the motion of the individual molecules is random in every direction, we do not detect any motion. If we enclose the gas within a container, we detect a pressure in the gas from the molecules colliding with the walls of our container. We can put the walls of our container anywhere inside the gas, and the force per area (the pressure) is the same. We can shrink the size of our "container" down to an infinitely small point, and the pressure has a single value at that point. Therefore, pressure is a scalar quantity, not a vector quantity. It has a magnitude but no direction associated with it. Pressure acts in all directions at a point inside a gas. At the surface of a gas, the pressure force acts perpendicular to the surface. ==Hydrostatic pressure== Hydrostatic pressure is the pressure due to the weight of a fluid. : where: *''rho'' (rho) is the density of the fluid *''Gee'' is the acceleration due to gravity *''h'' is the height of fluid above the point being measured Also see Pascal's Law. ==Stagnation pressure== Stagnation pressure is the pressure a fluid exerts when it is motionless. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure. Static and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See Bernoulli's equation. The pressure of a moving fluid can be measured using a Pitot probe, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure ''static pressure'' or ''stagnation pressure''. ==Units== The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m-2 or kg·s-2·m-1). This special name for the unit was added in 1971; before that, pressures in SI were expressed in units such as N/m² Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (PSI) and the bar (unit). The cgs unit of pressure is barye (ba). It is equal to 1 dyn·cm-2. Pressure is still sometimes expressed in kgf/cm² or g/cm² (often as kg/cm² and g/cm² without properly identifying the force units). The technical atmosphere (symbol: at) is 1 kgf/cm². In the United States air pressure is still measured in inHg — inches of mercury (element) (as in the mercury barometer). Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, because it gives the same numbers as the older millibar (mbar). Blood pressure is still measured in torr in most of the world, and lung pressures in centimeters of water are still common. These obsolete manometric units of pressure on the pressure exerted by the weight of some "standard" fluid under some "standard" gravity. They are effectively attempts to define a unit for expressing the readings of a manometer. When millimetres or inches of mercury are used today, they have precise definitions which can be expressed exactly in terms of SI units, though there were considerable minor variations in earlier usage. The water-based units depend on the density of water, a measured rather than defined quantity. The standard atmosphere (atm) is an established constant. It is approximately equal to typical air pressures at sea level and defined to be :standard atmosphere = 101 325 pascal = 101.325 kPa = 1013.25 hPa. A rule of thumb commonly used by Scuba diving is that one atmosphere is approximately equal to the pressure exerted by ten metres of water. Non-SI units presently or formerly in use include the following: *Atmosphere (unit)s *Manometric units: **Centimetres, inches and torr **Millimetres, CmH2O, metres, inches and feet of water *Customary and Foot-pound-second system of units units: **kip (unit)s, tons-force (short), tons-force (long), pounds-force, ounces-force, and poundals per square inch **Pounds-force, tons-force(short) and tons-force (long) per square foot *Non-SI metric units: **Bar (unit) and millibars **Kilograms-force (kiloponds), grams-force, tonnes-force (metric tons-force), newtons and dynes per square centimetre **Baryes = dyn/cm² and technical atmospheres = kgf/cm² **Kilograms-force and tonnes-force per square metre ===Conversion table===
==See also==
*Partial pressure
*Kinetic theory#Pressure
*Atmospheric pressure
*Sound pressure
*Microphone
*Timeline of temperature and pressure measurement technology
*Conversion of units
*Blood pressure
*:Category:Units of pressure
*Shear
== External links ==
*[http://calc.skyrocket.de/en/ Online Unit Converter - Conversion of many different units]
*[http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?An_Exercise_In_Air_Pressure An Exercise in Air Pressure]
*[http://www.grc.nasa.gov/WWW/K-12/airplane/pressure.html Pressure being a scalar quantity]
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Pressure can also be psychological, political, etc.; see also peer pressure.
Diving
Meteorology
Physical quantity
Thermodynamics
ms:Tekanan
==Meanings== I would say that pressure is the application of force upon a static body. Once the body moves, it is being pushed. In gravity-free and atmosphere-free space, no further pushing is required for the body to keep its inertial motion. : Good point, but that's more of a dicdef than an encyclopaedia entry. User:Jnc User_talk:Jnc 12:17, 6 Dec 2004 (UTC) To send an electric signal through a push-button, as in an elevator, the button is pushed all the way until it causes some pressure for the electric contact, unless it has been designed to act as a switch. --User:Ghitis 15:49, 12 Aug 2004 ==Formula== Something's wierd about that formula. A vector divided by a scalar is still a vector. So the formular does indeed give a vector. (And it should also be expressed as a limit as A goes to 0, too. That avoids the problem where A is not flat, and so potentially the total resultant F on A is less than the fluid pressure times A.) I think the problem is that we need to distinguish between the pressure exerted on the wall of a container, which is indeed a force (i.e. a vector, the limit of F/A), and the pressure ''inside'' the fluid, which is a scalar. They have the same ''numeric'' value (not to mention units), which is what is sometimes confusing, but are physically separate concepts. User:Jnc User_talk:Jnc 12:17, 6 Dec 2004 (UTC) Oh, the right way to define the scalar pressure (at least for gasses, liquids are of course different) is to talk about it as a combination of the density of the gas (i.e. number of molecules per unit volume) together with its temperature (which is effectively a measure of the velocity of those particles), and bring in the gas laws. User:Jnc User_talk:Jnc 12:25, 6 Dec 2004 (UTC) : OK, fair warning - if someone doesn't like this, say so, because "shortly" I will edit the article to reflect these ideas. User:Jnc User_talk:Jnc 00:30, 16 Dec 2004 (UTC) ==Atmosphere== I removed the unqualified statement that use of the atm as a unit "should be avoided"; it's not a great unit for most scientific work, but there are times when it's a very good choice. For instance, in scuba diving it's quite a handy unit because it matches the usual baseline pressure, making it easy to calculate effects on gas volumes etc ('V2 = V1/P2' is simpler than 'V2 = V1*P1/P2'), and the "+10 metres = +1 atm" rule adds a simple relationship between depth and pressure. In this sort of application, precision is less important than simplicity of use. --User:Calair 01:09, 26 Dec 2004 (UTC) :The problem with the use of atmospheres as a unit of measure has nothing to do with precision, and only a little to do with ambiguity. It has to do with the fact that these are not, and cannot be, units in the International System of units, and are unlikely to ever be listed as units acceptable for use with SI or temporarily acceptable for use with SI. :For the approximation you discuss, the accuracy of the approximation is very much dependent upon factors such as whether this is sea water or salt water, and temperature. For that purpose, 0.01 MPa/m, or 100 m = 1 MPa, is just as good. :: Some practical considerations about mental arithmetic: (1) The further the magnitude of your numbers is from 1, the more potential for error. (2) The more people have to work with fractions rather than whole numbers, the more potential for error. (3) The more operations you require someone to make, the more potential for error. (4) The more 'constants' that have to be remembered, the more potential for error. :: As a simple example, let's suppose somebody wants to figure out the proportional volume change of his BCD as he goes from the surface to a depth of 30m (useful for gauging what will happen to his buoyancy as he descends). :: Using atm/10 metre rule: depth = 30, 30/10 = 3 => pressure = 1+3=4 => proportional volume of BCD = 1/4. (I'm deliberately omitting units because that's how people generally conduct mental calculations, regardless of scientific correctness.) The only 'constant' that needs to be retrieved from memory is the 10 m/1 atm rule, and that only once. :: Under SI: depth = 30. Pressure = 0.01 x 30 (or alternately, 30/100; either way, we have to retrieve a constant from memory at this point) = 0.3, plus atmospheric pressure 0.1 (retrieve a second constant from memory) = 0.4, so proportional volume = 0.1 (retrieve second constant again)/0.4 = 1/4. :: Compare those two calculations. At every step of the way, the second is more prone to error. 30/10 is easier than 30/100 or 0.01 x 30; 1+3 is easier than 0.1 + 0.3; 1/4 is easier than 0.1/0.4. The first calculation only requires one 'constant' to be retrieved, once; the second requires two to be retrieved, one of them twice. :: For a scientist working at the surface with a calculator, SI is wonderful. It's rigorously defined, it's 'scientifically correct', etc etc. As a professional scientist, I insist on working in SI and the many who don't are a source of perpetual annoyance to me. But when we're talking about non-professionals performing mental calculations in an unfamiliar environment, while undergoing physical exertion and quite possibly 'drunk' from nitrogen narcosis, the simpler method is to be favoured - even if it makes it harder for them to tell a scientist what they've done when they get home. --User:Calair 00:25, 28 Dec 2004 (UTC) :"Should be avoided" was a pretty mild way of putting it. Maybe I should come up with some stronger language before I reinstate it. User:Gene Nygaard 04:55, 27 Dec 2004 (UTC) So what? Not everyone is a scientist. As long as there are significant communities of people for whom "atmosphere" is a useful, much-used, and well-understood unit, it's POV to call it "should be avoided". User:Jnc User_talk:Jnc 16:58, 27 Dec 2004 (UTC) :My wording could probably be cleaned up, but making clear what the standards-setters say does not violate NPOV. If it did, the mere classification into "SI units" and "other units" would also violate NPOV. ::I would have absolutely no objection to noting that it's non-SI and so should be avoided in scientific use. But that objection applies equally to all units in the non-SI section; as such it should be stated at the beginning of the section, clearly applying to all, rather than as a note attached only to atm. As it was, that proscription was not stated with a context of "in scientific use", making it unjustifiably broad. --User:Calair 00:25, 28 Dec 2004 (UTC) :I notice also your failure to maintain the same laissez-faire attitude towards grams force. Or maybe that was just an oversight on your part? User:Gene Nygaard 17:25, 27 Dec 2004 (UTC) :: I think you've misread my [http://en.wikipedia.org/w/index.php?title=Pressure&diff=8472174&oldid=8472151 | edit] there. As much as I loathe grams-force and such units, they are at least a legitimate unit of force and thus grams-force per cm^2 are a legitimate unit of pressure. The edit referred specifically to ''grams'' per cm^2 - not grams-force - and this usage is downright wrong. Not because it's not SI, but because it's not even dimensionally correct; grams and grams-force are not the same thing. --User:Calair 00:25, 28 Dec 2004 (UTC) If you want to say that it's not an SI unit, and use of atmosphere in scientific settings is therefore non desirable (I see it used in engineering stuff all the time, e.g. NASA spacecraft documentation), I have no problem with that. As to the "grams force", I have no idea what you're talking about. Are you confusing me with someone else? User:Jnc User_talk:Jnc 17:46, 27 Dec 2004 (UTC) :I mainly want to distinguish two different uses: a standard atmosphere as a constant, which is acceptable for use with SI, versus a standard atmosphere as a unit of measure, which is not. Perhaps the "standard model of the atmosphere" (not sure if that's the best name for this more involved concept) needs to be distinguished as well? :Yes, it was User:Calair, not you, who took out my comment about atmospheres being unacceptable, while not making a similar change to my comments about the 'g cm-2' units (often seen as "kg/cm²" pressure gauges) no longer being acceptable. :Curiously, the same User:Calair who took out my incorrect usage comment was the same one who inserted language about 'g·cm²', saying that "this usage is incorrect and should be avoided." After I clarified this, Calair left my statement untouched: "those formerly acceptable grams force are not a part of the modern SI and should be avoided." That was, however, a more detailed explanation than my statement about the use of the atmosphere as a unit of measure, so I concur in the need to revise my statement in that regard. User:Gene Nygaard 18:11, 27 Dec 2004 (UTC) :: I left it untouched because it's a busy time of year and I got distracted. Your statement does need to be clarified, because the problem with "g/cm^2" is not that "grams-force" aren't SI but that "grams" are not "grams-force"; the second half is correct but non-sequitur. :: My two edits are better seen as unrelated. The point of the 'atm' one was that although atms are not a SI unit of pressure, there are occasions when non-SI units are appropriate. The point of the other was that "g/cm^2" are not a legitimate unit for pressure at all, because "g" is not a unit of force. I would not have made the same objection to "grams-force/cm^2"; I don't like those units at all, but at least they're not outright *wrong*. --User:Calair 00:25, 28 Dec 2004 (UTC) :::That argument applies equally to pounds-force, tons-force, and the like. Make the same point about them, if you make it about grams-force (note that you specifically disclaimed the fact that grams-force are not SI as a reason for objection, so there is absolutely no other difference involved here). User:Gene Nygaard 06:52, 17 Jan 2005 (UTC) :::: Actually, there is a difference - claiming that "pounds are not a unit of force" is vastly more likely to trigger time-wasting arguments than making the same claim about grams. Further, since I have much more use for and interest in metric units than pounds & derivatives, I'm not the person to correct the latter. If somebody else wants to do it, great; in the meantime, better to correct some of the article than none of it.--User:Calair 12:06, 18 Jan 2005 (UTC) ==Stagnation pressure== : ''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? --User:Calair 22:49, 12 Mar 2005 (UTC) ==Problems with example== ''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 [http://www.batspeed.com/messageboard/6515.html]) 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 [http://www.trailerboats.com/site_page_1483/article_page_306.cfm] (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. --User:Calair 01:57, 13 Apr 2005 (UTC)
See other meanings of words starting from letter:
Words begining with Pressure:
Pressure
Pressure
Pressure-adapted_bacteria
Pressure-fed_cycle_(rocket)
Pressure-treated_lumber
Pressure-treated_wood
Pressures
Pressure_(physics)
Pressure_altitude
Pressure_barrier_osmosis
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Pressure_Chief
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Pressure_Cooker
Pressure_cooker
Pressure_cooking
Pressure_cooking
Pressure_flaking
Pressure_flow_hypothesis
Pressure_frying
Pressure_gauge
Pressure_gauge
Pressure_gradient_force
Pressure_group
Pressure_groups_in_the_United_Kingdom
Pressure_lamp
Pressure_point
Pressure_points
Pressure_release_valve
Pressure_relief_valve
Pressure_sensitive
Pressure_sensor
Pressure_sickness
Pressure_sore
Pressure_sores
Pressure_suit
Pressure_system
Pressure_system
Pressure_tank
Pressure_treated_lumber
Pressure_treated_wood
Pressure_ulcer
Pressure_vessel
Pressure_wave
Pressure
Pressure is the application of force to a surface, and the concentration of that force in a given area. A finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a thumbtack can easily damage the wall, even though the force applied is the same, because the point concentrates that force into a smaller area. More formally, pressure (symbol: ''p'' or ''P'') is the measure of the surface normal component of force (physics) that acts on a unit area, see also stress (physics): : where: *''p'' is the pressure *''F'' is the normal component of the force *''A'' is the area Often ''F'' is taken to be the magnitude of the mean vector force normal to the surface of area ''A'' upon which it exerts; the "surface" not necessarily being a that of a body, but for example the cross sectional area of a conduit. The gradient of pressure is force density. Pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure; such measurements are sometimes called gauge pressure. An example of this is the air pressure in a tire of a car, which might be said to be "thirty PSI", but is actually thirty PSI above atmospheric pressure. In technical work, this is often written as "30 PSIG" or, more commonly, "30 psig", though other methods which avoid attaching this information to the unit of pressure are preferred. [http://physics.nist.gov/Pubs/SP811/sec07.html#7.4 1] In the human body, pressure is measured by baroreceptors. "Pressure is a scalar quantity, but teachers and authors do not appear to believe this in their hearts." (McClelland, 1987) ==Scalar quantity== Let us look at a static gas; one that does not appear to move or flow. While the gas as a whole does not appear to move, the individual molecules of the gas, which we cannot see, are in constant random motion. Because we are dealing with a nearly infinite number of molecules and because the motion of the individual molecules is random in every direction, we do not detect any motion. If we enclose the gas within a container, we detect a pressure in the gas from the molecules colliding with the walls of our container. We can put the walls of our container anywhere inside the gas, and the force per area (the pressure) is the same. We can shrink the size of our "container" down to an infinitely small point, and the pressure has a single value at that point. Therefore, pressure is a scalar quantity, not a vector quantity. It has a magnitude but no direction associated with it. Pressure acts in all directions at a point inside a gas. At the surface of a gas, the pressure force acts perpendicular to the surface. ==Hydrostatic pressure== Hydrostatic pressure is the pressure due to the weight of a fluid. : where: *''rho'' (rho) is the density of the fluid *''Gee'' is the acceleration due to gravity *''h'' is the height of fluid above the point being measured Also see Pascal's Law. ==Stagnation pressure== Stagnation pressure is the pressure a fluid exerts when it is motionless. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure. Static and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See Bernoulli's equation. The pressure of a moving fluid can be measured using a Pitot probe, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure ''static pressure'' or ''stagnation pressure''. ==Units== The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m-2 or kg·s-2·m-1). This special name for the unit was added in 1971; before that, pressures in SI were expressed in units such as N/m² Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (PSI) and the bar (unit). The cgs unit of pressure is barye (ba). It is equal to 1 dyn·cm-2. Pressure is still sometimes expressed in kgf/cm² or g/cm² (often as kg/cm² and g/cm² without properly identifying the force units). The technical atmosphere (symbol: at) is 1 kgf/cm². In the United States air pressure is still measured in inHg — inches of mercury (element) (as in the mercury barometer). Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, because it gives the same numbers as the older millibar (mbar). Blood pressure is still measured in torr in most of the world, and lung pressures in centimeters of water are still common. These obsolete manometric units of pressure on the pressure exerted by the weight of some "standard" fluid under some "standard" gravity. They are effectively attempts to define a unit for expressing the readings of a manometer. When millimetres or inches of mercury are used today, they have precise definitions which can be expressed exactly in terms of SI units, though there were considerable minor variations in earlier usage. The water-based units depend on the density of water, a measured rather than defined quantity. The standard atmosphere (atm) is an established constant. It is approximately equal to typical air pressures at sea level and defined to be :standard atmosphere = 101 325 pascal = 101.325 kPa = 1013.25 hPa. A rule of thumb commonly used by Scuba diving is that one atmosphere is approximately equal to the pressure exerted by ten metres of water. Non-SI units presently or formerly in use include the following: *Atmosphere (unit)s *Manometric units: **Centimetres, inches and torr **Millimetres, CmH2O, metres, inches and feet of water *Customary and Foot-pound-second system of units units: **kip (unit)s, tons-force (short), tons-force (long), pounds-force, ounces-force, and poundals per square inch **Pounds-force, tons-force(short) and tons-force (long) per square foot *Non-SI metric units: **Bar (unit) and millibars **Kilograms-force (kiloponds), grams-force, tonnes-force (metric tons-force), newtons and dynes per square centimetre **Baryes = dyn/cm² and technical atmospheres = kgf/cm² **Kilograms-force and tonnes-force per square metre ===Conversion table===
| Pascal | bar | N/mm2 | kp/m2 | kp/cm2 (=1 at) | atm | torr | |
|---|---|---|---|---|---|---|---|
| 1 Pa (N/m2)= | 1 | 10-5 | 10-6 | 0.102 | 0.102×10-4 | 0.987×10-5 | 0.0075 |
| 1 bar (unit) (daN/cm2) = | 105 | 1 | 0.1 | 10,200 | 1.02 | 0.987 | 750 |
| 1 N/mm2 = | 106 | 10 | 1 | 1.02×105 | 10.2 | 9.87 | 7,501 |
| 1 kp/m2 = | 9.81 | 9.81×10-5 | 9.81×10-6 | 1 | 10-4 | 0.968×10-4 | 0.0736 |
| 1 kp/cm2 (1 at) = | 98,100 | 0.981 | 0.0981 | 10,000 | 1 | 0.968 | 736 |
| 1 atmospheric pressure (760 torr) = | 101,325 | 1.013 | 0.1013 | 10,330 | 1.033 | 1 | 760 |
| 1 torr (mmHg) = | 133 | 0.00133 | 1.33×10-4 | 13.6 | 0.00132 | 0.00132 | 1 |
Pressure
==Meanings== I would say that pressure is the application of force upon a static body. Once the body moves, it is being pushed. In gravity-free and atmosphere-free space, no further pushing is required for the body to keep its inertial motion. : Good point, but that's more of a dicdef than an encyclopaedia entry. User:Jnc User_talk:Jnc 12:17, 6 Dec 2004 (UTC) To send an electric signal through a push-button, as in an elevator, the button is pushed all the way until it causes some pressure for the electric contact, unless it has been designed to act as a switch. --User:Ghitis 15:49, 12 Aug 2004 ==Formula== Something's wierd about that formula. A vector divided by a scalar is still a vector. So the formular does indeed give a vector. (And it should also be expressed as a limit as A goes to 0, too. That avoids the problem where A is not flat, and so potentially the total resultant F on A is less than the fluid pressure times A.) I think the problem is that we need to distinguish between the pressure exerted on the wall of a container, which is indeed a force (i.e. a vector, the limit of F/A), and the pressure ''inside'' the fluid, which is a scalar. They have the same ''numeric'' value (not to mention units), which is what is sometimes confusing, but are physically separate concepts. User:Jnc User_talk:Jnc 12:17, 6 Dec 2004 (UTC) Oh, the right way to define the scalar pressure (at least for gasses, liquids are of course different) is to talk about it as a combination of the density of the gas (i.e. number of molecules per unit volume) together with its temperature (which is effectively a measure of the velocity of those particles), and bring in the gas laws. User:Jnc User_talk:Jnc 12:25, 6 Dec 2004 (UTC) : OK, fair warning - if someone doesn't like this, say so, because "shortly" I will edit the article to reflect these ideas. User:Jnc User_talk:Jnc 00:30, 16 Dec 2004 (UTC) ==Atmosphere== I removed the unqualified statement that use of the atm as a unit "should be avoided"; it's not a great unit for most scientific work, but there are times when it's a very good choice. For instance, in scuba diving it's quite a handy unit because it matches the usual baseline pressure, making it easy to calculate effects on gas volumes etc ('V2 = V1/P2' is simpler than 'V2 = V1*P1/P2'), and the "+10 metres = +1 atm" rule adds a simple relationship between depth and pressure. In this sort of application, precision is less important than simplicity of use. --User:Calair 01:09, 26 Dec 2004 (UTC) :The problem with the use of atmospheres as a unit of measure has nothing to do with precision, and only a little to do with ambiguity. It has to do with the fact that these are not, and cannot be, units in the International System of units, and are unlikely to ever be listed as units acceptable for use with SI or temporarily acceptable for use with SI. :For the approximation you discuss, the accuracy of the approximation is very much dependent upon factors such as whether this is sea water or salt water, and temperature. For that purpose, 0.01 MPa/m, or 100 m = 1 MPa, is just as good. :: Some practical considerations about mental arithmetic: (1) The further the magnitude of your numbers is from 1, the more potential for error. (2) The more people have to work with fractions rather than whole numbers, the more potential for error. (3) The more operations you require someone to make, the more potential for error. (4) The more 'constants' that have to be remembered, the more potential for error. :: As a simple example, let's suppose somebody wants to figure out the proportional volume change of his BCD as he goes from the surface to a depth of 30m (useful for gauging what will happen to his buoyancy as he descends). :: Using atm/10 metre rule: depth = 30, 30/10 = 3 => pressure = 1+3=4 => proportional volume of BCD = 1/4. (I'm deliberately omitting units because that's how people generally conduct mental calculations, regardless of scientific correctness.) The only 'constant' that needs to be retrieved from memory is the 10 m/1 atm rule, and that only once. :: Under SI: depth = 30. Pressure = 0.01 x 30 (or alternately, 30/100; either way, we have to retrieve a constant from memory at this point) = 0.3, plus atmospheric pressure 0.1 (retrieve a second constant from memory) = 0.4, so proportional volume = 0.1 (retrieve second constant again)/0.4 = 1/4. :: Compare those two calculations. At every step of the way, the second is more prone to error. 30/10 is easier than 30/100 or 0.01 x 30; 1+3 is easier than 0.1 + 0.3; 1/4 is easier than 0.1/0.4. The first calculation only requires one 'constant' to be retrieved, once; the second requires two to be retrieved, one of them twice. :: For a scientist working at the surface with a calculator, SI is wonderful. It's rigorously defined, it's 'scientifically correct', etc etc. As a professional scientist, I insist on working in SI and the many who don't are a source of perpetual annoyance to me. But when we're talking about non-professionals performing mental calculations in an unfamiliar environment, while undergoing physical exertion and quite possibly 'drunk' from nitrogen narcosis, the simpler method is to be favoured - even if it makes it harder for them to tell a scientist what they've done when they get home. --User:Calair 00:25, 28 Dec 2004 (UTC) :"Should be avoided" was a pretty mild way of putting it. Maybe I should come up with some stronger language before I reinstate it. User:Gene Nygaard 04:55, 27 Dec 2004 (UTC) So what? Not everyone is a scientist. As long as there are significant communities of people for whom "atmosphere" is a useful, much-used, and well-understood unit, it's POV to call it "should be avoided". User:Jnc User_talk:Jnc 16:58, 27 Dec 2004 (UTC) :My wording could probably be cleaned up, but making clear what the standards-setters say does not violate NPOV. If it did, the mere classification into "SI units" and "other units" would also violate NPOV. ::I would have absolutely no objection to noting that it's non-SI and so should be avoided in scientific use. But that objection applies equally to all units in the non-SI section; as such it should be stated at the beginning of the section, clearly applying to all, rather than as a note attached only to atm. As it was, that proscription was not stated with a context of "in scientific use", making it unjustifiably broad. --User:Calair 00:25, 28 Dec 2004 (UTC) :I notice also your failure to maintain the same laissez-faire attitude towards grams force. Or maybe that was just an oversight on your part? User:Gene Nygaard 17:25, 27 Dec 2004 (UTC) :: I think you've misread my [http://en.wikipedia.org/w/index.php?title=Pressure&diff=8472174&oldid=8472151 | edit] there. As much as I loathe grams-force and such units, they are at least a legitimate unit of force and thus grams-force per cm^2 are a legitimate unit of pressure. The edit referred specifically to ''grams'' per cm^2 - not grams-force - and this usage is downright wrong. Not because it's not SI, but because it's not even dimensionally correct; grams and grams-force are not the same thing. --User:Calair 00:25, 28 Dec 2004 (UTC) If you want to say that it's not an SI unit, and use of atmosphere in scientific settings is therefore non desirable (I see it used in engineering stuff all the time, e.g. NASA spacecraft documentation), I have no problem with that. As to the "grams force", I have no idea what you're talking about. Are you confusing me with someone else? User:Jnc User_talk:Jnc 17:46, 27 Dec 2004 (UTC) :I mainly want to distinguish two different uses: a standard atmosphere as a constant, which is acceptable for use with SI, versus a standard atmosphere as a unit of measure, which is not. Perhaps the "standard model of the atmosphere" (not sure if that's the best name for this more involved concept) needs to be distinguished as well? :Yes, it was User:Calair, not you, who took out my comment about atmospheres being unacceptable, while not making a similar change to my comments about the 'g cm-2' units (often seen as "kg/cm²" pressure gauges) no longer being acceptable. :Curiously, the same User:Calair who took out my incorrect usage comment was the same one who inserted language about 'g·cm²', saying that "this usage is incorrect and should be avoided." After I clarified this, Calair left my statement untouched: "those formerly acceptable grams force are not a part of the modern SI and should be avoided." That was, however, a more detailed explanation than my statement about the use of the atmosphere as a unit of measure, so I concur in the need to revise my statement in that regard. User:Gene Nygaard 18:11, 27 Dec 2004 (UTC) :: I left it untouched because it's a busy time of year and I got distracted. Your statement does need to be clarified, because the problem with "g/cm^2" is not that "grams-force" aren't SI but that "grams" are not "grams-force"; the second half is correct but non-sequitur. :: My two edits are better seen as unrelated. The point of the 'atm' one was that although atms are not a SI unit of pressure, there are occasions when non-SI units are appropriate. The point of the other was that "g/cm^2" are not a legitimate unit for pressure at all, because "g" is not a unit of force. I would not have made the same objection to "grams-force/cm^2"; I don't like those units at all, but at least they're not outright *wrong*. --User:Calair 00:25, 28 Dec 2004 (UTC) :::That argument applies equally to pounds-force, tons-force, and the like. Make the same point about them, if you make it about grams-force (note that you specifically disclaimed the fact that grams-force are not SI as a reason for objection, so there is absolutely no other difference involved here). User:Gene Nygaard 06:52, 17 Jan 2005 (UTC) :::: Actually, there is a difference - claiming that "pounds are not a unit of force" is vastly more likely to trigger time-wasting arguments than making the same claim about grams. Further, since I have much more use for and interest in metric units than pounds & derivatives, I'm not the person to correct the latter. If somebody else wants to do it, great; in the meantime, better to correct some of the article than none of it.--User:Calair 12:06, 18 Jan 2005 (UTC) ==Stagnation pressure== : ''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? --User:Calair 22:49, 12 Mar 2005 (UTC) ==Problems with example== ''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 [http://www.batspeed.com/messageboard/6515.html]) 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 [http://www.trailerboats.com/site_page_1483/article_page_306.cfm] (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. --User:Calair 01:57, 13 Apr 2005 (UTC)
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Pressure
Pressure
Pressure-adapted_bacteria
Pressure-fed_cycle_(rocket)
Pressure-treated_lumber
Pressure-treated_wood
Pressures
Pressure_(physics)
Pressure_altitude
Pressure_barrier_osmosis
Pressure_chamber
Pressure_Chief
Pressure_Chief
Pressure_coefficient
Pressure_Cooker
Pressure_cooker
Pressure_cooking
Pressure_cooking
Pressure_flaking
Pressure_flow_hypothesis
Pressure_frying
Pressure_gauge
Pressure_gauge
Pressure_gradient_force
Pressure_group
Pressure_groups_in_the_United_Kingdom
Pressure_lamp
Pressure_point
Pressure_points
Pressure_release_valve
Pressure_relief_valve
Pressure_sensitive
Pressure_sensor
Pressure_sickness
Pressure_sore
Pressure_sores
Pressure_suit
Pressure_system
Pressure_system
Pressure_tank
Pressure_treated_lumber
Pressure_treated_wood
Pressure_ulcer
Pressure_vessel
Pressure_wave
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