# Dictionary Definition

swiftness n : a rate (usually rapid) at which something happens; "the project advanced with gratifying speed" [syn: speed, fastness]

# User Contributed Dictionary

## English

### Noun

1. the state of being swift

# 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|.
Note that "v" is the variable for speed.
Objects that move horizontally as well as vertically (such as aircraft) distinguish forward speed and climbing speed.

## Units

Units of speed include:
Mach 1 ≈ 343 ms-1 ≈ 1235 km/h ≈ 768 mph in dry air at sea-level pressure and 293 kelvin (See Speed of sound for more detail.)
c = 299,792,458 ms-1
• Other important conversions
1 m/s = 3.6 km/h
1 mph = 1.609 km/h
1 knot = 1.852 km/h = 0.514 ms-1
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 ms-1; 0.0036 km/h; 0.0023 mph.
• A brisk walk = 1.667 ms-1; 6 km/h; 3.75 mph.
• Olympic sprinters (average speed over 100 metres) = 10 ms-1; 36 km/h; 22.5 mph.
• Speed limit on a French autoroute = 36.111 ms-1; 130 km/h; 80 mph.
• Top cruising speed of a Boeing 747-8 = 290.947 ms-1; 1047.41 km/h; 650.83 mph; (officially Mach 0.85)
• Official air speed record = 980.278 ms-1; 3,529 km/h; 2,188 mph.
• Space shuttle on re-entry = 7,777.778 ms-1; 28,000 km/h; 17,500 mph.
• the speed of sound in air (Mach 1) is about 340 ms-1, and 1500 ms-1 in water
• Taipei 101 Observatory Elevator = 1010 m/min ; 16.667 ms-1 ; 60.6 km/h; 37.6 mph

## References

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