Fontem paginae Celeritas inspicere

'''Celeritas''' est quam celer res movit, mathematice [[magnitudo]] [[velocitas|velocitatis]] rei, unitatibus usitatis [[metrum|metrorum]] per [[secundum]] divisorum. 

In [[physica]] quidem celeritas correpondit [[distantia]]e instantaniae quam corpus quoddam transcurrit in [[tempus|tempore]]. Denotata ab <math>v</math>, celeritas rei igitur datur a formula
:<math>v = \frac {dx}{dt}</math>.
ubi dx est distantia differentialis traversa et ''dt'' est tempus differentiale huius motionis.

{{NexInt}}

*[[Celeritas luminis]]
*[[Celeritas soni]]
*[[Systema Internationale]]
<!--In many situations, objects do not move at a constant speed.  For example, if a car goes 60 miles in 2 hours, its average speed during that time is 30 miles per hour, but its instantaneous speed may have varied.  For an object which is [[Acceleration|accelerating or decelerating]], the instantaneous speed is given by
:<math>v = \left|\frac {dx}{dt}\right|</math>,
where <math>dx</math> is the distance it travels in a very short period of time <math>dt</math>.  If the object travels a total distance <math>x</math> in time <math>t</math>, its average speed over that time is given by
:<math>v = \left|\frac {d\vec{x}}{dt}\right|</math>.

==Units==
Units of speed include:
*[[meters per second]], (symbol m&nbsp;s<sup>−1</sup> or m/s), the [[SI derived unit]]
*[[kilometers per hour]], (symbol km/h)
*[[miles per hour]], (symbol mph)
*[[knot (nautical)|knot]]s ([[nautical mile]]s per hour, symbol kt)
*[[Mach number]], speed divided by the [[speed of sound]]
*[[speed of light]] in [[vacuum]] (symbol ''c'') is one of the [[natural units]]
::''c'' = 299,792,458&nbsp;m/s

Important conversions between units of speed include:
::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;m/s

 ==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>. 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==
{{main|Orders of magnitude (speed)}}
*Speed of a common [[snail]] = 0.001 m/s; 0.004 km/h; 0.002 mph (1 millimeter per second).
*A brisk [[walk]] = 1.7 m/s; 6 km/h; 4 mph (5.5 feet per second).
*[[Olympic Games|Olympic]] [[Sprint (race)|sprinters]] (average speed over 100 metres) = 10 m/s; 36 km/h; 22 mph.
*Speed limit on a [[France|French]] [[Freeway|autoroute]] = 36 m/s; 130 km/h; 80 mph.
*[[Taipei 101]] observatory elevator = 1010 m/min ; 16.7 m/s ; 60.6 km/h; 37.6 mph
*Cruising speed of a [[Boeing 747-8]] = 290 m/s; 1050 km/h; 650 mph; ([[Mach]] 0.85)
*The [[speed of sound]] in dry air at sea-level pressure and 20 °C (293 [[kelvin]]) = 343&nbsp;ms<sup>-1</sup> ≈ 1235&nbsp;km/h ≈ 768&nbsp;mph ( = Mach 1 by definition).
*Official [[flight airspeed record]] = 980 m/s; 3,530 km/h; 2,194 mph.
*[[Escape velocity]] on Earth: 11.2 km/s
*[[Space shuttle]] on re-entry = 7,800 m/s; 28,000 km/h; 17,500 mph.
*Average orbital speed of planet [[Earth]] = 29,783 m/s; 107,218 km/h; 66,623 mph.
*[[Speed of light]] in [[vacuum]] (symbol ''c'') 299,792,458&nbsp;m/s (exactly, by definition).
==See also==
{{wiktionarypar2|speed|swiftness}}
*[[Air speed]]
*[[Land speed]]
*[[List of vehicle speed records]]
*[[Projectile#Typical_projectile_speeds|Typical projectile speeds]]
*[[Speedometer]]
*[[V speeds]]
-->
==Fons==
* [[Richard Feynman|Richard P. Feynman]], Robert B. Leighton, Matthew Sands. [[The Feynman Lectures on Physics]], Volume I, Section 8-2. Addison-Wesley, Reading, Massachusetts (1963). ISBN 0-201-02116-1.  

==Nexus Externi==
*[http://www.unitsconversion.com.ar/speedunitsconversion/ Programa quod inter unitates celeritatis convertit]
*[http://kph.com Converte kph ad mph] 

[[Categoria:Physica]]

{{Myrias|Physica}}