Dictionary Definition
swiftness n : a rate (usually rapid) at which
something happens; "the project advanced with gratifying speed"
[syn: speed, fastness]
User Contributed Dictionary
Extensive Definition
Speed is the rate of motion,
or equivalently the rate of change in position, often expressed as
distance d traveled per
unit
of time t.
Speed is a scalar
quantity with dimensions distance/time; the equivalent 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
component of velocity.
In mathematical notation, it is simply:
 v = \left\frac \right.
Objects that move horizontally as well as
vertically (such as aircraft) distinguish forward speed
and climbing
speed.
Units
Units of speed include: meters per second, (symbol ms1; m/s), the SI derived unit
 kilometers per hour, (symbol km/h)
 miles per hour, (symbol m/h)
 knots (nautical miles per hour, symbol kt)
 Mach, where Mach 1 is the speed of sound; Mach n is n times as fast.

 Mach 1 ≈ 343 ms1 ≈ 1235 km/h ≈ 768 mph in dry air at sealevel 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 ms1
 Other important conversions

 1 m/s = 3.6 km/h
 1 mph = 1.609 km/h
 1 knot = 1.852 km/h = 0.514 ms1
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 \tilde), 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:
 \tilde = \frac
Instantaneous speed defined as a function of
time on interval [t_0, t_1]
gives average speed:
 \tilde = \frac
while instant speed defined as a function of
distance (or length) on interval [l_0, l_1]
gives average speed:
 \tilde = \frac
It is often intuitively expected, but incorrect,
that going half a distance with speed v_ and second half with
speed v_, produces total average speed \tilde = \frac. The
correct value is \tilde = \frac (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
distribution function (either in time or on distance):
 v \sim D_t\; \Rightarrow \; \tilde = \int v D_t(v) \, dv
 v \sim D_l\; \Rightarrow \; \tilde = \frac
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 ms1; 0.0036 km/h; 0.0023 mph.
 A brisk walk = 1.667 ms1; 6 km/h; 3.75 mph.
 Olympic sprinters (average speed over 100 metres) = 10 ms1; 36 km/h; 22.5 mph.
 Speed limit on a French autoroute = 36.111 ms1; 130 km/h; 80 mph.
 Top cruising speed of a Boeing 7478 = 290.947 ms1; 1047.41 km/h; 650.83 mph; (officially Mach 0.85)
 Official air speed record = 980.278 ms1; 3,529 km/h; 2,188 mph.
 Space shuttle on reentry = 7,777.778 ms1; 28,000 km/h; 17,500 mph.
 the speed of sound in air (Mach 1) is about 340 ms1, and 1500 ms1 in water
 Taipei 101 Observatory Elevator = 1010 m/min ; 16.667 ms1 ; 60.6 km/h; 37.6 mph
See also
External Links
References
 Weisstein, Eric W. "Speed." From MathWorldA Wolfram Web Resource. http://mathworld.wolfram.com/Speed.html
swiftness in Afrikaans: Spoed
swiftness in Arabic: سرعة
swiftness in Catalan: Rapidesa
swiftness in Welsh: Buanedd
swiftness in Danish: Fart
swiftness in German: Geschwindigkeit
swiftness in Estonian: Kiirus
swiftness in Spanish: Rapidez
swiftness in Esperanto: Rapido
swiftness in French: Vitesse
swiftness in Korean: 속력
swiftness in Croatian: Brzina
swiftness in Ido: Rapideso
swiftness in Icelandic: Hraði
swiftness in Italian: Velocità media
swiftness in Hebrew: מהירות
swiftness in Georgian: სიჩქარე
swiftness in Latvian: Ātrums
swiftness in Lithuanian: Greitis
swiftness in Macedonian: Брзина
swiftness in Malay (macrolanguage): Laju
swiftness in Dutch: Snelheid
swiftness in Japanese: 速さ
swiftness in Norwegian: Fart
swiftness in Norwegian Nynorsk: Fart
swiftness in Polish: Szybkość
swiftness in Portuguese: Rapidez
swiftness in Romanian: Viteză
swiftness in Russian: Скорость
swiftness in Simple English: Speed
swiftness in Finnish: Vauhti
swiftness in Swedish: Fart
swiftness in Thai: อัตราเร็ว
swiftness in Ukrainian: Швидкість
swiftness in Chinese: 速率
Synonyms, Antonyms and Related Words
abruptness, agility, air speed, alacrity, bat, brevity, briefness, briskness, celerity, decisiveness, dispatch, expedition, expeditiousness,
fastness, feverishness, flight, flit, flurry, furiousness, gait, ground speed, haste, hastiness, hurriedness, hurry, hustle, immediacy, immediateness, impetuosity, impetuousness, impulsiveness, instantaneousness,
knots, lightning speed,
miles per hour, nimbleness, pace, precipitance, precipitancy, precipitation, precipitousness,
promptitude,
promptness, punctuality, punctualness, quickness, rapidity, rashness, readiness, round pace,
rpm, rush, rustle, sharpness, shortness, smartness, snappiness, speed, speediness, spryness, suddenness, summariness, swift rate,
velocity