Speed 28748 223217993 2008-07-03T02:47:21Z Autobogg 3320491 /* Examples of different speeds */ {{refimprove|date=April 2007}} {{otheruses1|speed in physics}} '''Speed''' is the rate of [[Motion (physics)|motion]], or equivalently the rate of change in position, often expressed as [[distance]] ''d'' traveled per [[Units of measurement|unit]] of [[time]] ''t''. Speed is a [[Scalar (physics)|scalar]] [[quantity]] with [[dimension]]s [[length|distance]]/[[time]]; the equivalent [[vector (spatial)|vector]] quantity to speed is known as [[velocity]]. Speed is measured in the same physical units of measurement as velocity, but does not contain the element of direction that velocity has. Speed is thus the [[magnitude (mathematics)|magnitude]] component of velocity. In mathematical notation, it is simply: :<math>v = \left|\frac {d}{t}\right|.</math> Note that "v" is the variable for speed. Objects that move horizontally as well as vertically (such as [[aircraft]]) distinguish '''[[V speeds|forward speed]]''' and '''[[V speeds|climbing speed]]'''. ==Units== Units of speed include: *[[meters per second]], (symbol ms<sup>-1</sup>; m/s), the [[SI derived unit]] *[[kilometers per hour]], (symbol km/h) *[[miles per hour]], (symbol m/h) *[[knot (nautical)|knot]]s ([[nautical mile]]s per hour, symbol kt) *[[Mach number|Mach]], where Mach 1 is the [[speed of sound]]; Mach ''n'' is ''n'' times as fast. ::Mach 1 ≈ 343&nbsp;ms<sup>-1</sup> ≈ 1235&nbsp;km/h ≈ 768&nbsp;mph in dry air at sea-level pressure and 293 [[kelvin]] (See ''[[Speed of sound]]'' for more detail.) *[[speed of light]] in [[vacuum]] (symbol ''c'') is one of the [[natural units]] ::''c'' = 299,792,458&nbsp;ms<sup>-1</sup> *Other important conversions ::1 m/s = 3.6&nbsp;km/h ::1 mph = 1.609&nbsp;km/h ::1 knot = 1.852&nbsp;km/h = 0.514&nbsp;ms<sup>-1</sup> Vehicles often have a [[speedometer]] to measure the speed they are going. ==Average speed== Speed as a [[physical property]] represents primarily instantaneous speed. In real life we often use ''average speed'' (denoted <math>|\tilde{v}|</math>), which is [[rate]] of total [[distance]] (or [[length]]) and [[time]] interval. For example, if you go 60 miles in 2 hours, your ''average'' speed during that time is 60/2 = 30 miles per hour, but your instantaneous speed may have varied. In mathematical notation: :<math>|\tilde{v}| = \frac{\Delta l}{\Delta t}</math> Instantaneous speed defined as a function of [[time]] on interval <math>[t_0, t_1]</math> gives average speed: :<math>|\tilde{v}| = \frac{\int_{t_0}^{t_1} |v|(t) \, dt}{\Delta t}</math> while instantaneous speed defined as a function of [[distance]] (or [[length]]) on interval <math>[l_0, l_1]</math> gives average speed: :<math>|\tilde{v}| = \frac{\Delta l}{\int_{l_0}^{l_1} \frac{1}{|v|(l)} \, dl}</math> It is often intuitively expected, but incorrect, that going half a distance with speed <math>|v|_{a}</math> and second half with speed <math>|v|_{b}</math>, produces total average speed <math>|\tilde{v}| = \frac{|v|_a + |v|_b}{2}</math>. The correct value is <math>|\tilde{v}| = \frac{2}{\frac{1}{|v|_a} + \frac{1}{|v|_b}}</math><br /> (Note that the first is a proper [[arithmetic mean]] while the second is a proper [[harmonic mean]]). Average speed can be derived also from speed [[Probability density function|distribution function]] (either in time or on distance): :<math>|v| \sim D_t\; \Rightarrow \; |\tilde{v}| = \int |v| D_t(|v|) \, dv</math> :<math>|v| \sim D_l\; \Rightarrow \; |\tilde{v}| = \frac{1}{\int \frac{D_l(|v|)}{|v|} \, dv}</math> == Examples of different speeds == Below are some examples of different speed. See also main article [[Orders of magnitude (speed)]]: *Speed of a common [[snail]] = 0.001 ms<sup>-1</sup>; 0.0036 km/h; 0.0023 mph (1.02 millimeters per second). *A brisk [[walk]] = 1.667 ms<sup>-1</sup>; 6 km/h; 3.75 mph (5.5 feet per second). *[[Olympic Games|Olympic]] [[Sprint (race)|sprinters]] (average speed over 100 metres) = 10 ms<sup>-1</sup>; 36 km/h; 22.5 mph. * [[Taipei 101]] observatory elevator = 1010 m/min ; 16.667 ms<sup>-1</sup> ; 60.6 km/h; 37.6 mph *Speed limit on a [[France|French]] [[Freeway|autoroute]] = 36.111 ms<sup>-1</sup>; 130 km/h; 80 mph. *Top cruising speed of a [[Boeing 747-8]] = 290.947 ms<sup>-1</sup>; 1047.41 km/h; 650.83 mph; ([[Mach]] 0.85) *the [[speed of sound]] in dry air at 20 °C (68 °F) is 343 ms<sup>-1</sup> ; 1235 km/h, or 770 mph. *Official [[air speed record]] = 980.278 ms<sup>-1</sup>; 3,529 km/h; 2,188 mph. *[[Space shuttle]] on re-entry = 7,777.778 ms<sup>-1</sup>; 28,000 km/h; 17,500 mph. *Average orbital speed of planet [[Earth]] = 29,783 ms<sup>-1</sup>; 107,218.8 km/h; 66,622.67 mph. ==See also== {{Wiktionary|swiftness}} *[[Air speed]] *[[Land speed]] *[[List of vehicle speed records]] *[[Orders of magnitude (speed)]] *[[Paul Virilio]] *{{ml|Projectile|Typical_projectile_speeds|Typical projectile speeds}} *[[V speeds]] ==External Links== *[http://www.unitsconversion.com.ar/speedunitsconversion/ Online speed units conversion] ==References== *Weisstein, Eric W. "Speed." From MathWorld--A Wolfram Web Resource. [http://mathworld.wolfram.com/Speed.html] {{Kinematics}} [[Category:Units of velocity|Units of velocity]] [[Category:Physical quantity]] [[af:Spoed]] [[ar:سرعة]] [[ca:Rapidesa]] [[cy:Buanedd]] [[da:Fart]] [[de:Geschwindigkeit]] [[et:Kiirus]] [[es:Rapidez]] [[eo:Rapido]] [[fr:Vitesse]] [[ko:속력]] [[hr:Brzina]] [[io:Rapideso]] [[is:Hraði]] [[it:Velocità media]] [[he:מהירות]] [[ka:სიჩქარე]] [[lv:Ātrums]] [[lt:Greitis]] [[mk:Брзина]] [[ms:Laju]] [[nl:Snelheid]] [[ja:速さ]] [[no:Fart]] [[nn:Fart]] [[pl:Szybkość]] [[pt:Rapidez]] [[ro:Viteză]] [[ru:Скорость]] [[simple:Speed]] [[fi:Vauhti]] [[sv:Fart]] [[th:อัตราเร็ว]] [[uk:Швидкість]] [[zh:速率]]