text:
The Three Little Pigs
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For other uses, see Three Little Pigs (disambiguation).
The Three Little Pigs
The wolf blows down the straw house in a 1904 adaptation of the story. Illustration by Leonard Leslie Brooke.
Folk tale
Name
The Three Little Pigs
Data
Aarne-Thompson grouping
124
Country
England
Related
'
The Three Little Pigs is a fable about three pigs who build three houses of different materials. A big bad wolf blows down the first two pigs' houses, made of straw and sticks respectively, but is unable to destroy the third pig's house, made of bricks. Printed versions date back to the 1840s, but the story itself is thought to be much older. The phrases used in the story, and the various morals drawn from it, have become embedded in Western culture. Many versions of The Three Little Pigs have been recreated or have been modified over the years, sometimes making the wolf a kind character. It is a type 124 folktale in the Aarne–Thompson classification system.
Contents
[hide]
1 Traditional versions
2 Later adaptations
2.1 Disney cartoon
2.2 Warner Brothers and MGM versions
2.3 Subsequent retellings
3 References
4 External links
Traditional versions[edit]
Illustration from J. Jacobs, English Fairy Tales (New York, 1895)
The Three Little Pigs was included in The Nursery Rhymes of England (London and New York, c.1886), by James Halliwell-Phillipps.[1] The story in its arguably best-known form appeared in English Fairy Tales by Joseph Jacobs, first published in 1890 and crediting Halliwell as his source.[2] The story begins with the title characters being sent out into the world by their mother, to "seek out their fortune". The first little pig builds a house of straw, but a wolf blows it down and devours him. The second little pig builds a house of sticks, which the wolf also blows down, and the second little pig is also devoured. Each exchange between wolf and pig features ringing proverbial phrases, namely:
"Little pig, little pig, let me come in."
"not by the hair on my chinny chin chin."
"Then I'll huff, and I'll puff, and I'll blow your house down."[3]
The third little pig builds a house of bricks. The wolf fails to blow down the house. He then attempts to trick the pig out of the house by asking to meet him at various places, but he is outwitted each time. Finally, the wolf resolves to come down the chimney, whereupon the pig catches the wolf in a cauldron of boiling water, slams the lid on, then cooks and eats him. In some versions, the first and second little pigs are not eaten by the wolf after he demolishes their homes, but instead run to their brother's house, and after the wolf goes down the chimney he either dies like in the original, or runs away and never returns to eat the three little pigs, who all survive in either case.
The story uses the literary rule of three, expressed in this case as a "contrasting three", as the third pig's brick house turns out to be the only one which is adequate to withstand the wolf.[4] Variations of the tale appeared in Uncle Remus: His Songs and Sayings in 1881. The story also made an appearance in Nights with Uncle Remus in 1883, both by Joel Chandler Harris, in which the pigs were replaced by Brer Rabbit. Andrew Lang included it in The Green Fairy Book, published in 1892, but did not cite his source. In contrast to Jacobs's version, which left the pigs nameless, Lang's retelling cast the pigs as Browny, Whitey, and Blacky. It also set itself apart by exploring each pig's character and detailing interaction between them. The antagonist of this version is a fox, not a wolf. The pigs' houses are made either of mud, cabbage, or brick. Blacky, the third pig, rescues his brother and sister from the fox's den after the fox has been defeated.
Later adaptations[edit]
Disney cartoon[edit]
Main article: The Three Little Pigs (film)
The most well-known version of the story is an award-winning 1933 Silly Symphony cartoon, which was produced by Walt Disney. The production cast the title characters as Fifer Pig, Fiddler Pig, and Practical Pig. The first two are depicted as both frivolous and arrogant. The story has been somewhat softened. The first two pigs still get their houses blown down, but escape from the wolf. Also, the wolf is not boiled to death but simply burns his behind and runs away. Three sequels soon followed in 1934, 1936 and 1939 respectively.
Fifer Pig, Fiddler Pig, Practical Pig and the Big Bad Wolf appeared in the 2001 series Disney's House of Mouse in many episodes, and again in Mickey's Magical Christmas: Snowed in at the House of Mouse. The three pigs can be seen in Walt Disney Parks and Resorts as greetable characters.[5]
Warner Brothers and MGM versions[edit]
In 1942, there was a Merrie Melodies version made that was a serious musical treatment, plus the usual Friz Freleng visual humor. It parodies both the Disney version, and Fantasia itself.
Other versions of the tale were also made. One was an MGM Tex Avery cartoon named Blitz Wolf, a 1942 wartime version with the Wolf as a Nazi. Another animated spoof was a 1952 Warner Brothers cartoon called The Turn-Tale Wolf, directed by Robert McKimson. This cartoon tells the story from the wolf's point of view and makes the pigs out to be the villains. Another Warner Brothers spoof was Friz Freleng's The Three Little Bops (1957), which depicts the three little pigs as jazz musicians who refuse to let the wolf join their band.
Subsequent retellings[edit]
In 1953, Tex Avery did a Droopy cartoon, "The Three Little Pups". In it, the wolf is a Southern-speaking dog catcher (voiced by Daws Butler) trying to catch Droopy and his brothers, Snoopy and Loopy, to put in the dog pound. Though successfully blowing the first two houses down, he meets his match when he fails to blow down Droopy's house of bricks. The dog catcher makes several failed attempts to destroy the house and catch the pups. His last failed attempt ended with him "going to television" where he is playing a cowboy on the TV show the pups were watching.
The 1989 parody, The True Story of the 3 Little Pigs!, is presented as a first-person narrative by the wolf, who portrays the entire incident as a misunderstanding; he had gone to the pigs to borrow some sugar, had destroyed their houses in a sneezing fit, ate the first two pigs to not waste food (since they'd died in the house collapse anyway), and was caught attacking the third pig's house after the pig had continually insulted him.[2]
The 1992 Green Jellö song, Three Little Pigs (and its claymation music video) sets the story in Los Angeles. The wolf drives a Harley Davidson motorcycle, the first little pig is an aspiring guitarist, the second is a cannabis smoking, dumpster diving evangelist and the third holds a Master of Architecture degree from Harvard University. In the end, with all three pigs barricaded in the brick house, the third pig calls 9-1-1. John Rambo is dispatched to the scene, and kills the wolf with a machine gun.
The 1993 children's book The Three Little Wolves and the Big Bad Pig inverts the cast and makes a few changes to the plot: the wolves build a brick house, then a concrete house, then a steel house, and finally a house of flowers. The pig is unable to blow the houses down, destroying them by other means, but eventually gives up his wicked ways when he smells the scent of the flower house, and becomes friends with the wolves.
The three pigs and the wolf appear in the four Shrek films, and the specials Shrek the Halls and Scared Shrekless.
In 2003, the Flemish company Studio 100 created a musical called Three Little Pigs (Dutch: De 3 Biggetjes), which follows the three daughters of the pig with the house of stone with new original songs, introducing a completely new story loosely based on the original story. The musical was specially written for the band K3, who play the three little pigs, Pirky, Parky and Porky (Dutch: Knirri, Knarri and Knorri).
In 2014, Peter Lund let the three little pigs live together in a village in the musical Grimm with Little Red Riding Hood and other fairy tale characters.
In 2015, Saturn Animation Studio, Canada, produced an interactive adaptation of "Three Little Pigs" for all mobile devices.[6]
References[edit]
Jump up ^ Ashliman, Professor D. L. "Three Little Pigs and other folktales of Aarne-Thompson-Uther type 124". Folklore and Mythology Electronic Texts. University of Pittsburgh. Retrieved 25 July 2010.
^ Jump up to: a b Tatar, Maria (2002). The Annotated Classic Fairy Tales. W. W. Norton & Company. pp. 206–211. ISBN 978-0-393-05163-6.
Jump up ^ Jacobs, Joseph (1890). English Fairy Tales. Oxford University. pp. 68–72.
Jump up ^ Booker, Christopher (2005). "The Rule of Three". The Seven Basic Plots: Why We Tell Stories. Continuum International Publishing Group. pp. 230–231.
Jump up ^ Waldman, Steven (November 1996). "In search of the real three little pigs - different versions of the story 'The Three Little Pigs'". Washington Monthly. [permanent dead link]
Jump up ^ Three Little Pigs, by Saturn Animation
External links[edit]
Wikisource has original text related to this article:
The Three Little Pigs
Wikimedia Commons has media related to Three little pigs.
The nursery rhymes of England By James Orchard Halliwell-Phillipps, pp. 37–41 on books.google.com
Three Little Pigs fairy tale with an interactive twist
The Story of the Three Little Pigs in The nursery rhymes of England, by Halliwell, pp. 37–41 on Archive.org
The Three Little Pigs: Pleasure principle versus reality principle, from: The Uses of Enchantment, The Meaning and Importance of Fairy Tales, by Bruno Bettelheim, Vintage Books, NY, 1975
19th century versions of the Three Little Pigs story
The Golden Goose Book: The Story of the Three Little Pigs from Google books
Three Little Pigs The fairy tale, lushly illustrated in The Colorful Story Book of 1941.
MP3 of the song "Who's Afraid of the Big Bad Wolf"
The Adventures of the Three Little Pigs, interactive Book"
The Three Little Pigs at The Big Cartoon DataBase
[show]
v
t
e
Pigs
Kingdom Animalia
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Subfamily Suinae
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[show]
v
t
e
The Three Little Pigs
Film
The Three Little Pigs (1933)
The Big Bad Wolf (1934)
Three Little Wolves (1936)
The Practical Pig (1939)
Unstable Fables (2008)
Stage
De 3 Biggetjes (2003 musical)
Parody films
Blitz Wolf (1942)
Pigs in a Polka (1943)
The Windblown Hare (1949)
Three Little Bops (1957)
Parody books
The True Story of the 3 Little Pigs! (1989)
The Three Little Wolves and the Big Bad Pig (1993)
The Three Pigs (2001)
Games
Oink!
Songs
"Who's Afraid of the Big Bad Wolf?" (1933)
"Three Little Pigs" (1991)
Story in story
Season's Greetings
Related
San zhi xiao zhu
Big Bad Wolf (character)
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text:
Mach number
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"Mach 2" redirects here. For the film, see Mach 2 (film).
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An F/A-18 Hornet creating a vapor cone at transonic speed just before reaching the speed of sound
In fluid dynamics, the Mach number (M or Ma) (/mɑːx/; German: [maχ]) is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound.[1][2]
M = u c , {\displaystyle \mathrm {M} ={\frac {u}{c}},}
where:
M is the Mach number,
u is the local flow velocity with respect to the boundaries (either internal, such as an object immersed in the flow, or external, like a channel), and
c is the speed of sound in the medium.
By definition, Mach 1 is equal to the speed of sound. Mach 0.65 is 65% of the speed of sound (subsonic), and Mach 1.35 is 35% faster than the speed of sound (supersonic).
The local speed of sound, and thereby the Mach number, depends on the condition of the surrounding medium, in particular the temperature. The Mach number is primarily used to determine the approximation with which a flow can be treated as an incompressible flow. The medium can be a gas or a liquid. The boundary can be traveling in the medium, or it can be stationary while the medium flows along it, or they can both be moving, with different velocities: what matters is their relative velocity with respect to each other. The boundary can be the boundary of an object immersed in the medium, or of a channel such as a nozzle, diffusers or wind tunnels channeling the medium. As the Mach number is defined as the ratio of two speeds, it is a dimensionless number. If M < 0.2–0.3 and the flow is quasi-steady and isothermal, compressibility effects will be small and simplified incompressible flow equations can be used.[1][2]
The Mach number is named after Austrian physicist and philosopher Ernst Mach, and is a designation proposed by aeronautical engineer Jakob Ackeret. As the Mach number is a dimensionless quantity rather than a unit of measure, with Mach, the number comes after the unit; the second Mach number is "Mach 2" instead of "2 Mach" (or Machs). This is somewhat reminiscent of the early modern ocean sounding unit "mark" (a synonym for fathom), which was also unit-first, and may have influenced the use of the term Mach. In the decade preceding faster-than-sound human flight, aeronautical engineers referred to the speed of sound as Mach's number, never "Mach 1."[3]
Contents
[hide]
1 Overview
2 Classification of Mach regimes
3 High-speed flow around objects
4 High-speed flow in a channel
5 Calculation
5.1 Calculating Mach number from pitot tube pressure
6 See also
7 Notes
8 External links
Overview[edit]
Mach number is useful because the fluid behaves in a similar manner at a given Mach number, regardless of other variables.[4] As modeled in the International Standard Atmosphere, dry air at mean sea level, standard temperature of 15 °C (59 °F), the speed of sound is 340.3 meters per second (1,116.5 ft/s).[5] The speed of sound is not a constant; in a gas, it increases as the absolute temperature increases, and since atmospheric temperature generally decreases with increasing altitude between sea level and 11,000 meters (36,089 ft), the speed of sound also decreases. For example, the standard atmosphere model lapses temperature to −56.5 °C (−69.7 °F) at 11,000 meters (36,089 ft) altitude, with a corresponding speed of sound (Mach 1) of 295.0 meters per second (967.8 ft/s), 86.7% of the sea level value.
Classification of Mach regimes[edit]
While the terms "subsonic" and "supersonic," in the purest sense, refer to speeds below and above the local speed of sound respectively, aerodynamicists often use the same terms to talk about particular ranges of Mach values. This occurs because of the presence of a "transonic regime" around M = 1 where approximations of the Navier-Stokes equations used for subsonic design actually no longer apply; the simplest explanation is that the flow locally begins to exceed M = 1 even though the freestream Mach number is below this value.
Meanwhile, the "supersonic regime" is usually used to talk about the set of Mach numbers for which linearised theory may be used, where for example the (air) flow is not chemically reacting, and where heat-transfer between air and vehicle may be reasonably neglected in calculations.
In the following table, the "regimes" or "ranges of Mach values" are referred to, and not the "pure" meanings of the words "subsonic" and "supersonic".
Generally, NASA defines "high" hypersonic as any Mach number from 10 to 25, and re-entry speeds as anything greater than Mach 25. Aircraft operating in this regime include the Space Shuttle and various space planes in development.
Regime
Mach
knots
mph
km/h
m/s
General plane characteristics
Subsonic
<0.8 <530 <609 <980 <273 Most often propeller-driven and commercial turbofan aircraft with high aspect-ratio (slender) wings, and rounded features like the nose and leading edges.
The subsonic speed range is that range of speeds within all of the airflow over an aircraft is less than Mach 1. The critical Mach number (Mcrit) is lowest free stream mach number at which airflow over any part of the aircraft first reaches Mach 1. So the subsonic speed range includes all speeds that are less than Mcrit.
Transonic
0.8-1.3 530-794 609-914 980-1,470 273-409 Transonic aircraft nearly always have swept wings, causing the delay of drag-divergence, and often feature a design that adheres to the principles of the Whitcomb Area rule.
The transonic speed range is that range of speeds within which the airflow over different parts of an aircraft is between subsonic and supersonic. So the regime of flight from Mcrit up to Mach 1.3 is called the transonic range.
Supersonic
1.3–5.0 794-3,308 915-3,806 1,470–6,126 410–1,702 The supersonic speed range is that range of speeds within which all of the airflow over an aircraft is supersonic (more than Mach 1). But airflow meeting the leading edges is initially decelerated, so the free stream speed must be slightly greater than Mach 1 to ensure that all of the flow over the aircraft is supersonic. It is commonly accepted that the supersonic speed range starts at a free stream speed greater than Mach 1.3.
Aircraft designed to fly at supersonic speeds show large differences in their aerodynamic design because of the radical differences in the behaviour of flows above Mach 1. Sharp edges, thin aerofoil-sections, and all-moving tailplane/canards are common. Modern combat aircraft must compromise in order to maintain low-speed handling; "true" supersonic designs include the F-104 Starfighter, SR-71 Blackbird and BAC/Aérospatiale Concorde.
Hypersonic
5.0–10.0 3,308-6,615 3,806–7,680 6,126–12,251 1,702–3,403 The X-15, at Mach 6.72 is one of the fastest manned aircraft. Also, cooled nickel-titanium skin; highly integrated (due to domination of interference effects: non-linear behaviour means that superposition of results for separate components is invalid), small wings, such as those on the Mach 5 X-51A Waverider
High-hypersonic
10.0–25.0 6,615-16,537 7,680–19,031 12,251–30,626 3,403–8,508 The NASA X-43, at Mach 9.6 is one of the fastest aircraft. Thermal control becomes a dominant design consideration. Structure must either be designed to operate hot, or be protected by special silicate tiles or similar. Chemically reacting flow can also cause corrosion of the vehicle's skin, with free-atomic oxygen featuring in very high-speed flows. Hypersonic designs are often forced into blunt configurations because of the aerodynamic heating rising with a reduced radius of curvature.
Re-entry speeds
>25.0 >16,537 >19,031 >30,626 >8,508 Ablative heat shield; small or no wings; blunt shape
High-speed flow around objects[edit]
Flight can be roughly classified in six categories:
Regime
Subsonic
Transonic
Sonic
Supersonic
Hypersonic
Hypervelocity
Mach
<0.8 0.8–1.2 1.0 1.2–5.0 5.0–10.0 >10.0
For comparison: the required speed for low Earth orbit is approximately 7.5 km/s = Mach 25.4 in air at high altitudes.
At transonic speeds, the flow field around the object includes both sub- and supersonic parts. The transonic period begins when first zones of M > 1 flow appear around the object. In case of an airfoil (such as an aircraft's wing), this typically happens above the wing. Supersonic flow can decelerate back to subsonic only in a normal shock; this typically happens before the trailing edge. (Fig.1a)
As the speed increases, the zone of M > 1 flow increases towards both leading and trailing edges. As M = 1 is reached and passed, the normal shock reaches the trailing edge and becomes a weak oblique shock: the flow decelerates over the shock, but remains supersonic. A normal shock is created ahead of the object, and the only subsonic zone in the flow field is a small area around the object's leading edge. (Fig.1b)
(a) (b)
Fig. 1. Mach number in transonic airflow around an airfoil; M < 1 (a) and M > 1 (b).
When an aircraft exceeds Mach 1 (i.e. the sound barrier), a large pressure difference is created just in front of the aircraft. This abrupt pressure difference, called a shock wave, spreads backward and outward from the aircraft in a cone shape (a so-called Mach cone). It is this shock wave that causes the sonic boom heard as a fast moving aircraft travels overhead. A person inside the aircraft will not hear this. The higher the speed, the more narrow the cone; at just over M = 1 it is hardly a cone at all, but closer to a slightly concave plane.
At fully supersonic speed, the shock wave starts to take its cone shape and flow is either completely supersonic, or (in case of a blunt object), only a very small subsonic flow area remains between the object's nose and the shock wave it creates ahead of itself. (In the case of a sharp object, there is no air between the nose and the shock wave: the shock wave starts from the nose.)
As the Mach number increases, so does the strength of the shock wave and the Mach cone becomes increasingly narrow. As the fluid flow crosses the shock wave, its speed is reduced and temperature, pressure, and density increase. The stronger the shock, the greater the changes. At high enough Mach numbers the temperature increases so much over the shock that ionization and dissociation of gas molecules behind the shock wave begin. Such flows are called hypersonic.
It is clear that any object traveling at hypersonic speeds will likewise be exposed to the same extreme temperatures as the gas behind the nose shock wave, and hence choice of heat-resistant materials becomes important.
High-speed flow in a channel[edit]
As a flow in a channel becomes supersonic, one significant change takes place. The conservation of mass flow rate leads one to expect that contracting the flow channel would increase the flow speed (i.e. making the channel narrower results in faster air flow) and at subsonic speeds this holds true. However, once the flow becomes supersonic, the relationship of flow area and speed is reversed: expanding the channel actually increases the speed.
The obvious result is that in order to accelerate a flow to supersonic, one needs a convergent-divergent nozzle, where the converging section accelerates the flow to sonic speeds, and the diverging section continues the acceleration. Such nozzles are called de Laval nozzles and in extreme cases they are able to reach hypersonic speeds (Mach 13 (15,926 km/h; 9,896 mph) at 20 °C).
An aircraft Machmeter or electronic flight information system (EFIS) can display Mach number derived from stagnation pressure (pitot tube) and static pressure.
Calculation[edit]
The Mach number at which an aircraft is flying can be calculated by
M = u c {\displaystyle \mathrm {M} ={\frac {u}{c}}}
where:
M is the Mach number
u is velocity of the moving aircraft and
c is the speed of sound at the given altitude
Note that the dynamic pressure can be found as:
q = γ 2 p M 2 {\displaystyle q={\frac {\gamma }{2}}p\,\mathrm {M} ^{2}}
Assuming air to be an ideal gas, the formula to compute Mach number in a subsonic compressible flow is derived from Bernoulli's equation for M < 1:[6]
M = 2 γ − 1 [ ( q c p + 1 ) γ − 1 γ − 1 ] {\displaystyle \mathrm {M} ={\sqrt {{\frac {2}{\gamma -1}}\left[\left({\frac {q_{c}}{p}}+1\right)^{\frac {\gamma -1}{\gamma }}-1\right]}}\,}
where:
qc is impact pressure (dynamic pressure) and
p is static pressure
γ {\displaystyle \ \gamma \,} is the ratio of specific heat of a gas at a constant pressure to heat at a constant volume (1.4 for air).
The formula to compute Mach number in a supersonic compressible flow is derived from the Rayleigh supersonic pitot equation:
p t p = [ γ + 1 2 M 2 ] ( γ γ − 1 ) ⋅ [ γ + 1 ( 1 − γ + 2 γ M 2 ) ] ( 1 γ − 1 ) {\displaystyle {\frac {p_{t}}{p}}=\left[{\frac {\gamma +1}{2}}\mathrm {M} ^{2}\right]^{\left({\frac {\gamma }{\gamma -1}}\right)}\cdot \left[{\frac {\gamma +1}{\left(1-\gamma +2\gamma \,\mathrm {M} ^{2}\right)}}\right]^{\left({\frac {1}{\gamma -1}}\right)}}
Calculating Mach number from pitot tube pressure[edit]
Mach number is a function of temperature and true airspeed. Aircraft flight instruments, however, operate using pressure differential to compute Mach number, not temperature.
Assuming air to be an ideal gas, the formula to compute Mach number in a subsonic compressible flow is found from Bernoulli's equation for M < 1 (above):[6]
M = 5 [ ( q c p + 1 ) 2 7 − 1 ] {\displaystyle \mathrm {M} ={\sqrt {5\left[\left({\frac {q_{c}}{p}}+1\right)^{\frac {2}{7}}-1\right]}}\,}
The formula to compute Mach number in a supersonic compressible flow can be found from the Rayleigh supersonic pitot equation (above) using parameters for air:
M = 0.88128485 ( q c p + 1 ) ( 1 − 1 7 M 2 ) 2.5 {\displaystyle \mathrm {M} =0.88128485{\sqrt {\left({\frac {q_{c}}{p}}+1\right)\left(1-{\frac {1}{7\,\mathrm {M} ^{2}}}\right)^{2.5}}}}
where:
qc is the dynamic pressure measured behind a normal shock.
As can be seen, M appears on both sides of the equation. The easiest method to solve the supersonic M calculation is to enter both the subsonic and supersonic equations into a computer spreadsheet, or an equivalent program to solve it numerically. It is first determined whether M is indeed greater than 1.0 by calculating M from the subsonic equation. If M is greater than 1.0 at that point, then the value of M from the subsonic equation is used as the initial condition in the supersonic equation. Then a simple iteration of the supersonic equation is performed, each time using the last computed value of M, until M converges to a value—usually in just a few iterations.[6] Alternatively, Newton's method can also be used.
See also[edit]
Critical Mach number
Machmeter
Ramjet
Scramjet
Speed of sound
True airspeed
Notes[edit]
^ Jump up to: a b Young, Donald F.; Bruce R. Munson; Theodore H. Okiishi; Wade W. Huebsch (2010). A Brief Introduction to Fluid Mechanics (5 ed.). John Wiley & Sons. p. 95. ISBN 978-0-470-59679-1.
^ Jump up to: a b Graebel, W.P. (2001). Engineering Fluid Mechanics. Taylor & Francis. p. 16. ISBN 978-1-56032-733-2.
Jump up ^ Bodie, Warren M., The Lockheed P-38 Lightning, Widewing Publications ISBN 0-9629359-0-5.
Jump up ^ Nancy Hall (ed.). "Mach Number". NASA.
Jump up ^ Clancy, L.J. (1975), Aerodynamics, Table 1, Pitman Publishing London, ISBN 0-273-01120-0
^ Jump up to: a b c Olson, Wayne M. (2002). "AFFTC-TIH-99-02, Aircraft Performance Flight Testing." (PDF). Air Force Flight Test Center, Edwards AFB, CA, United States Air Force. Archived September 4, 2011, at the Wayback Machine.
External links[edit]
Gas Dynamics Toolbox Calculate Mach number and normal shock wave parameters for mixtures of perfect and imperfect gases.
NASA's page on Mach Number Interactive calculator for Mach number.
NewByte standard atmosphere calculator and speed converter
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Computer file
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This article is about computer files and file systems in general terms. For a more detailed and technical article, see File system.
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A computer file is a computer resource for recording data discretely in a computer storage device. Just as words can be written to paper, so can information be written to a computer file.
There are different types of computer files, designed for different purposes. A file may be designed to store a picture, a written message, a video, a computer program, or a wide variety of other kinds of data. Some types of files can store several types of information at once.
By using computer programs, a person can open, read, change, and close a computer file. Computer files may be reopened, modified, and copied an arbitrary number of times.
Typically, files are organised in a file system, which keeps track of where the files are located on disk and enables user access.
Contents
[hide]
1 Etymology
2 File contents
2.1 File size
2.2 Organization of data in a file
2.3 Operations
3 Identifying and organizing
4 Protection
5 Storage
6 Back up
7 File systems and file managers
8 See also
9 Notes
10 External links
Etymology[edit]
A punched card file
The twin disk files of an IBM 305 system
The word "file" derives from the Latin filum ("a thread").[1]
"File" was used in the context of computer storage as early as January 1940. In Punched Card Methods in Scientific Computation[2], W.J. Eckert stated, "The first extensive use of the early Hollerith Tabulator in astronomy was made by Comrie[3]. He used it for building a table from successive differences, and for adding large numbers of harmonic terms". "Tables of functions are constructed from their differences with great efficiency, either as printed tables or as a file of punched cards."
In February 1950: In a Radio Corporation of America (RCA) advertisement in Popular Science Magazine[4] describing a new "memory" vacuum tube it had developed, RCA stated: "the results of countless computations can be kept 'on file' and taken out again. Such a 'file' now exists in a 'memory' tube developed at RCA Laboratories. Electronically it retains figures fed into calculating machines, holds them in storage while it memorizes new ones - speeds intelligent solutions through mazes of mathematics."
In 1952, "file" denoted, inter alia, information stored on punched cards.[5]
In early use, the underlying hardware, rather than the contents stored on it, was denominated a "file". For example, the IBM 350 disk drives were denominated "disk files".[6] The introduction, circa 1961, by the Burroughs MCP and the MIT Compatible Time-Sharing System of the concept of a "file system" that managed several virtual "files" on one storage device is the origin of the contemporary denotation of the word. Although the contemporary "register file" demonstrates the early concept of files, its use has greatly decreased.
File contents[edit]
On most modern operating systems, files are organized into one-dimensional arrays of bytes. The format of a file is defined by its content since a file is solely a container for data, although, on some platforms the format is usually indicated by its filename extension, specifying the rules for how the bytes must be organized and interpreted meaningfully. For example, the bytes of a plain text file (.txt in Windows) are associated with either ASCII or UTF-8 characters, while the bytes of image, video, and audio files are interpreted otherwise. Most file types also allocate a few bytes for metadata, which allows a file to carry some basic information about itself.
Some file systems can store arbitrary (not interpreted by the file system) file-specific data outside of the file format, but linked to the file, for example extended attributes or forks. On other file systems this can be done via sidecar files or software-specific databases. All those methods, however, are more susceptible to loss of metadata than are container and archive file formats.
File size[edit]
Main article: File size
At any instant in time, a file might have a size, normally expressed as number of bytes, that indicates how much storage is associated with the file. In most modern operating systems the size can be any non-negative whole number of bytes up to a system limit. Many older operating systems kept track only of the number of blocks or tracks occupied by a file on a physical storage device. In such systems, software employed other methods to track the exact byte count (e.g., CP/M used a special control character, Ctrl-Z, to signal the end of text files).
The general definition of a file does not require that its size have any real meaning, however, unless the data within the file happens to correspond to data within a pool of persistent storage. A special case is a zero byte file; these files can be newly created files that have not yet had any data written to them, or may serve as some kind of flag in the file system, or are accidents (the results of aborted disk operations). For example, the file to which the link /bin/ls points in a typical Unix-like system probably has a defined size that seldom changes. Compare this with /dev/null which is also a file, but its size may be obscure. (This is misleading because /dev/null is not really a file: in Unix-like systems, all resources, including devices, are accessed like files, but there is still a real distinction between files and devices—at core, they behave differently—and the obscurity of the "size" of /dev/null is one manifestation of this. As a character device, /dev/null has no size.)
Organization of data in a file[edit]
Information in a computer file can consist of smaller packets of information (often called "records" or "lines") that are individually different but share some common traits. For example, a payroll file might contain information concerning all the employees in a company and their payroll details; each record in the payroll file concerns just one employee, and all the records have the common trait of being related to payroll—this is very similar to placing all payroll information into a specific filing cabinet in an office that does not have a computer. A text file may contain lines of text, corresponding to printed lines on a piece of paper. Alternatively, a file may contain an arbitrary binary image (a BLOB) or it may contain an executable.
The way information is grouped into a file is entirely up to how it is designed. This has led to a plethora of more or less standardized file structures for all imaginable purposes, from the simplest to the most complex. Most computer files are used by computer programs which create, modify or delete the files for their own use on an as-needed basis. The programmers who create the programs decide what files are needed, how they are to be used and (often) their names.
In some cases, computer programs manipulate files that are made visible to the computer user. For example, in a word-processing program, the user manipulates document files that the user personally names. Although the content of the document file is arranged in a format that the word-processing program understands, the user is able to choose the name and location of the file and provide the bulk of the information (such as words and text) that will be stored in the file.
Many applications pack all their data files into a single file called an archive file, using internal markers to discern the different types of information contained within. The benefits of the archive file are to lower the number of files for easier transfer, to reduce storage usage, or just to organize outdated files. The archive file must often be unpacked before next using.
Operations[edit]
The most basic operations that programs can perform on a file are:
Create a new file
Change the access permissions and attributes of a file
Open a file, which makes the file contents available to the program
Read data from a file
Write data to a file
Close a file, terminating the association between it and the program
Files on a computer can be created, moved, modified, grown, shrunk, and deleted. In most cases, computer programs that are executed on the computer handle these operations, but the user of a computer can also manipulate files if necessary. For instance, Microsoft Word files are normally created and modified by the Microsoft Word program in response to user commands, but the user can also move, rename, or delete these files directly by using a file manager program such as Windows Explorer (on Windows computers) or by command lines (CLI).
In Unix-like systems, user-space programs do not operate directly, at a low level, on a file. Only the kernel deals with files, and it handles all user-space interaction with files in a manner that is transparent to the user-space programs. The operating system provides a level of abstraction, which means that interaction with a file from user-space is simply through its filename (instead of its filehandle). For example, rm filename will not delete the file itself, but only a link to the file. There can be many links to a file, but when they are all removed, the kernel considers that file's memory space free to be reallocated. This free space is commonly considered a security risk (due to the existence of file recovery software). Any secure-deletion program uses kernel-space (system) functions to wipe the file's data.
Identifying and organizing[edit]
Files and folders arranged in a hierarchy
In modern computer systems, files are typically accessed using names (filenames). In some operating systems, the name is associated with the file itself. In others, the file is anonymous, and is pointed to by links that have names. In the latter case, a user can identify the name of the link with the file itself, but this is a false analogue, especially where there exists more than one link to the same file.
Files (or links to files) can be located in directories. However, more generally, a directory can contain either a list of files or a list of links to files. Within this definition, it is of paramount importance that the term "file" includes directories. This permits the existence of directory hierarchies, i.e., directories containing sub-directories. A name that refers to a file within a directory must be typically unique. In other words, there must be no identical names within a directory. However, in some operating systems, a name may include a specification of type that means a directory can contain an identical name for more than one type of object such as a directory and a file.
In environments in which a file is named, a file's name and the path to the file's directory must uniquely identify it among all other files in the computer system—no two files can have the same name and path. Where a file is anonymous, named references to it will exist within a namespace. In most cases, any name within the namespace will refer to exactly zero or one file. However, any file may be represented within any namespace by zero, one or more names.
Any string of characters may or may not be a well-formed name for a file or a link depending upon the context of application. Whether or not a name is well-formed depends on the type of computer system being used. Early computers permitted only a few letters or digits in the name of a file, but modern computers allow long names (some up to 255 characters) containing almost any combination of unicode letters or unicode digits, making it easier to understand the purpose of a file at a glance. Some computer systems allow file names to contain spaces; others do not. Case-sensitivity of file names is determined by the file system. Unix file systems are usually case sensitive and allow user-level applications to create files whose names differ only in the case of characters. Microsoft Windows supports multiple file systems, each with different policies[which?] regarding case-sensitivity. The common FAT file system can have multiple files whose names differ only in case if the user uses a disk editor to edit the file names in the directory entries. User applications, however, will usually not allow the user to create multiple files with the same name but differing in case.
Most computers organize files into hierarchies using folders, directories, or catalogs. The concept is the same irrespective of the terminology used. Each folder can contain an arbitrary number of files, and it can also contain other folders. These other folders are referred to as subfolders. Subfolders can contain still more files and folders and so on, thus building a tree-like structure in which one "master folder" (or "root folder" — the name varies from one operating system to another) can contain any number of levels of other folders and files. Folders can be named just as files can (except for the root folder, which often does not have a name). The use of folders makes it easier to organize files in a logical way.
When a computer allows the use of folders, each file and folder has not only a name of its own, but also a path, which identifies the folder or folders in which a file or folder resides. In the path, some sort of special character—such as a slash—is used to separate the file and folder names. For example, in the illustration shown in this article, the path /Payroll/Salaries/Managers uniquely identifies a file called Managers in a folder called Salaries, which in turn is contained in a folder called Payroll. The folder and file names are separated by slashes in this example; the topmost or root folder has no name, and so the path begins with a slash (if the root folder had a name, it would precede this first slash).
Many (but not all) computer systems use extensions in file names to help identify what they contain, also known as the file type. On Windows computers, extensions consist of a dot (period) at the end of a file name, followed by a few letters to identify the type of file. An extension of .txt identifies a text file; a .doc extension identifies any type of document or documentation, commonly in the Microsoft Word file format; and so on. Even when extensions are used in a computer system, the degree to which the computer system recognizes and heeds them can vary; in some systems, they are required, while in other systems, they are completely ignored if they are presented.
Protection[edit]
Many modern computer systems provide methods for protecting files against accidental and deliberate damage. Computers that allow for multiple users implement file permissions to control who may or may not modify, delete, or create files and folders. For example, a given user may be granted only permission to read a file or folder, but not to modify or delete it; or a user may be given permission to read and modify files or folders, but not to execute them. Permissions may also be used to allow only certain users to see the contents of a file or folder. Permissions protect against unauthorized tampering or destruction of information in files, and keep private information confidential from unauthorized users.
Another protection mechanism implemented in many computers is a read-only flag. When this flag is turned on for a file (which can be accomplished by a computer program or by a human user), the file can be examined, but it cannot be modified. This flag is useful for critical information that must not be modified or erased, such as special files that are used only by internal parts of the computer system. Some systems also include a hidden flag to make certain files invisible; this flag is used by the computer system to hide essential system files that users should not alter.
Storage[edit]
Any file that has any useful purpose, must have some physical manifestation. That is, a file (an abstract concept) in a real computer system must have a real physical analogue if it is to exist at all.
In physical terms, most computer files are stored on some type of data storage device. For example, most operating systems store files on a hard disk. Hard disks have been the ubiquitous form of non-volatile storage since the early 1960s.[7] Where files contain only temporary information, they may be stored in RAM. Computer files can be also stored on other media in some cases, such as magnetic tapes, compact discs, Digital Versatile Discs, Zip drives, USB flash drives, etc. The use of solid state drives is also beginning to rival the hard disk drive.
In Unix-like operating systems, many files have no associated physical storage device. Examples are /dev/null and most files under directories /dev, /proc and /sys. These are virtual files: they exist as objects within the operating system kernel.
As seen by a running user program, files are usually represented either by a File control block or by a file handle. A File control block (FCB) is an area of memory which is manipulated to establish a filename etc. and then passed to the operating system as a parameter, it was used by older IBM operating systems and early PC operating systems including CP/M and early versions of MS-DOS. A file handle is generally either an opaque data type or an integer, it was introduced in around 1961 by the ALGOL-based Burroughs MCP running on the Burroughs B5000 but is now ubiquitous.
Back up[edit]
When computer files contain information that is extremely important, a back-up process is used to protect against disasters that might destroy the files. Backing up files simply means making copies of the files in a separate location so that they can be restored if something happens to the computer, or if they are deleted accidentally.
There are many ways to back up files. Most computer systems provide utility programs to assist in the back-up process, which can become very time-consuming if there are many files to safeguard. Files are often copied to removable media such as writable CDs or cartridge tapes. Copying files to another hard disk in the same computer protects against failure of one disk, but if it is necessary to protect against failure or destruction of the entire computer, then copies of the files must be made on other media that can be taken away from the computer and stored in a safe, distant location.
The grandfather-father-son backup method automatically makes three back-ups; the grandfather file is the oldest copy of the file and the son is the current copy.
File systems and file managers[edit]
The way a computer organizes, names, stores and manipulates files is globally referred to as its file system. Most computers have at least one file system. Some computers allow the use of several different file systems. For instance, on newer MS Windows computers, the older FAT-type file systems of MS-DOS and old versions of Windows are supported, in addition to the NTFS file system that is the normal file system for recent versions of Windows. Each system has its own advantages and disadvantages. Standard FAT allows only eight-character file names (plus a three-character extension) with no spaces, for example, whereas NTFS allows much longer names that can contain spaces. You can call a file "Payroll records" in NTFS, but in FAT you would be restricted to something like payroll.dat (unless you were using VFAT, a FAT extension allowing long file names).
File manager programs are utility programs that allow users to manipulate files directly. They allow you to move, create, delete and rename files and folders, although they do not actually allow you to read the contents of a file or store information in it. Every computer system provides at least one file-manager program for its native file system. For example, File Explorer (formerly Windows Explorer) is commonly used in Microsoft Windows operating systems, and Nautilus is common under several distributions of Linux.
See also[edit]
Block (data storage)
Computer file management
Data hierarchy
File camouflage
File copying
File conversion
File deletion
File directory
File manager
File system
Filename
Flat file database
Object composition
Soft copy
Notes[edit]
Jump up ^ "Online Etymology Dictionary".
Jump up ^ Eckert, W. J.; Thomas J. Watson Astronomical Computing Bureau, New York. (1940). Punched card methods in scientific computation. New York: The Thomas J. Watson Astronomical Computing Bureau, Columbia University.
Jump up ^ Comrie, L. J. (1928-04-13). "On the Construction of Tables by Interpolation.: (Plate 6.)". Monthly Notices of the Royal Astronomical Society. 88 (6): 506–523. doi:10.1093/mnras/88.6.506. ISSN 0035-8711.
Jump up ^ Popular Science Magazine, February 1950, page 96. Books.google.com. Retrieved 2014-03-07.
Jump up ^ Robert S. Casey, et al. Punched Cards: Their Applications to Science and Industry. 1952.
Jump up ^ Martin H. Weik. Ballistic Research Laboratories Report #1115. March 1961. pp. 314-331.
Jump up ^ Magnetic Storage Handbook 2nd Ed., Section 2.1.1, Disk File Technology, Mee and Daniel, (c)1990,
External links[edit]
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