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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:

- $v\; =\; left|frac\; \{d\}\{t\}right|.$

Objects that move horizontally as well as vertically (such as aircraft) distinguish forward speed and climbing speed.

- meters per second, (symbol ms
^{-1}; m/s), the SI derived unit - kilometers per hour, (symbol km/h)
- miles per hour, (symbol mph)
- 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 ms
^{-1}≈ 1235 km/h ≈ 768 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 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.

In mathematical notation:

- $|tilde\{v\}|\; =\; frac\{Delta\; l\}\{Delta\; t\}$

Instantaneous speed defined as a function of time on interval $[t\_0,\; t\_1]$ gives average speed:

- $|tilde\{v\}|\; =\; frac\{int\_\{t\_0\}^\{t\_1\}\; |v|(t)\; ,\; dt\}\{Delta\; t\}$

while instantaneous speed defined as a function of distance (or length) on interval $[l\_0,\; l\_1]$ gives average speed:

- $|tilde\{v\}|\; =\; frac\{Delta\; l\}\{int\_\{l\_0\}^\{l\_1\}\; frac\{1\}$
It is often intuitively expected, but incorrect, that going half a distance with speed $|v|\_\{a\}$ and second half with speed $|v|\_\{b\}$, produces total average speed $|tilde\{v\}|\; =\; frac$

. The correct value is $|tilde\{v\}|\; =\; frac\{2\}\{frac\{1\}$

(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):

- $|v|\; sim\; D\_t;\; Rightarrow\; ;\; |tilde\{v\}|\; =\; int\; |v|\; D\_t(|v|)\; ,\; dv$

- $|v|\; sim\; D\_l;\; Rightarrow\; ;\; |tilde\{v\}|\; =\; frac\{1\}\{int\; frac\{D\_l(|v|)\}$

> , dv}

- A brisk walk = 1.667 ms
^{-1}; 6 km/h; 3.75 mph (5.5 feet per second). - Average orbital speed of planet Earth = 29,783 ms
^{-1}; 107,218.8 km/h; 66,622.67 mph. - Official air speed record = 980.278 ms
^{-1}; 3,529 km/h; 2,188 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. - Speed of a common snail = 0.001 ms
^{-1}; 0.0036 km/h; 0.0023 mph (1.02 millimeters per second). - Taipei 101 observatory elevator = 1010 m/min ; 16.667 ms
^{-1}; 60.6 km/h; 37.6 mph - the speed of sound in dry air at 20 °C (68 °F) is 343 ms
^{-1}; 1235 km/h, or 770 mph. - Top speed of a Boeing 747-8 = 290.947 ms
^{-1}; 1047.41 km/h; 650.83 mph; (Mach 0.85) - Space shuttle on re-entry = 7,777.778 ms
^{-1}; 28,000 km/h; 17,500 mph.

- Air speed
- Land speed
- List of vehicle speed records
- Orders of magnitude (speed)
- Paul Virilio
- V speeds

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Last updated on Friday October 10, 2008 at 18:01:23 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Friday October 10, 2008 at 18:01:23 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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