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| {{about|the property of moving bodies|persons named Speed|Speed (name)|the film|Speed (1994 film)|other uses|Speed (disambiguation)}}
| | The writer is known by the title of Figures Wunder. Bookkeeping is what I do. North Dakota is where me and my spouse reside. To collect cash is what his family and him appreciate.<br><br>Here is my web site [http://U6.ro/healthyfooddelivery98296 http://U6.ro/healthyfooddelivery98296] |
| {{redirect|Slow|other uses|Slow (disambiguation)}}
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| {{Infobox physical quantity
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| |name = Speed
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| |image = [[File:Mersan.JPG|250px]]
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| |caption = Speed can be thought of as the rate at which an object covers [[distance]]. A fast-moving object has a high speed and covers a relatively large distance in a given amount of time, while a slow-moving object covers a relatively small amount of distance in the same amount of time.
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| |unit = m/s, m s<sup>−1</sup>
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| |symbols = ''v''
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| }}
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| {{Classical mechanics|cTopic=Fundamental concepts}}
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| In everyday use and in [[kinematics]], the '''speed''' of an object is the [[magnitude (mathematics)|magnitude]] of its [[velocity]] (the [[time derivative|rate of change]] of its [[Position (vector)|position]]); it is thus a [[Scalar (physics)|scalar]] quantity.<ref>{{cite book|last=Wilson|first=Edwin Bidwell|title=Vector analysis: a text-book for the use of students of mathematics and physics, founded upon the lectures of J. Willard Gibbs|date=1901|pages=125|url=http://hdl.handle.net/2027/mdp.39015000962285?urlappend=%3Bseq=149}} This is the likely origin of the speed/velocity terminology in vector physics.</ref> The '''average speed''' of an object in an interval of time is the [[distance]] travelled by the object divided by the [[Time|duration]] of the interval;<ref>{{cite web|title=Speed & Velocity|url=http://physics.info/velocity/}}</ref> the instantaneous speed is the [[limit (mathematics)|limit]] of the average speed as the duration of the time interval approaches zero.
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| Like velocity, speed has the [[dimensional analysis|dimension]]s of a [[length]] divided by a [[time]]; the [[International System of Units|SI unit]] of speed is the [[metre per second]], but the most usual unit of speed in everyday usage is the [[kilometre per hour]] or, in the US and the UK, [[miles per hour]]. For air and marine travel the [[Knot (unit)|knot]] is commonly used.
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| The fastest possible speed at which energy or information can travel, according to [[special relativity]], is the [[speed of light]] in a vacuum ''c'' = {{val|299792458}} metres per second (approximately {{val|1079000000|u=km/h}} or {{val|671000000|u=mph}}). [[Matter]] cannot quite reach the speed of light, as this would require an infinite amount of energy. In relativity physics, the concept of [[rapidity]] replaces the classical idea of speed.
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| ==Definition==
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| The Italian physicist [[Galileo Galilei]] is credited with being the first to measure speed by considering the distance covered and the time it takes. Galileo defined speed as the distance covered per unit of time.<ref name="Hewitt 2006, p. 42">Hewitt (2006), p. 42</ref> In equation form, this is
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| :<math>v = \frac{d}{t},</math>
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| where ''v'' is speed, ''d'' is distance, and ''t'' is time. A cyclist who covers 30 metres in a time of 2 seconds, for example, has a speed of 15 metres per second. Objects in motion often have variations in speed (a car might travel along a street at 50 km/h, slow to 0 km/h, and then reach 30 km/h). | |
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| In mathematical terms, the speed ''v'' is defined as the magnitude of the [[velocity]] '''v''', that is, the [[derivative]] of the position '''r''' with respect to [[time]]:
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| :<math>v = \left|\boldsymbol v\right| = \left|\dot {\boldsymbol r}\right| = \left|\frac{d\boldsymbol r}{dt}\right|\,.</math>
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| If ''s'' is the length of the path travelled until time ''t'', the speed equals the time derivative of ''s'':
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| :<math>v = \frac{ds}{dt}.</math>
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| In the special case where the velocity is constant (that is, constant speed in a straight line), this can be simplified to ''v'' = ''s''/''t''. The average speed over a finite time interval is the total distance travelled divided by the time duration.
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| ===Instantaneous speed===
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| By looking at a [[speedometer]], one can read the speed of a car at any instant, or its ''instantaneous speed''.<ref name="Hewitt 2006, p. 42"/> A car travelling at 50 km/h generally goes for less than one hour at a constant speed, but if it did go at that speed for a full hour, it would travel 50 km. If the vehicle continued at that speed for half an hour, it would cover half that distance (25 km). If it continued for only one minute, it would cover about 833 m.
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| ===Average speed===
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| Different from instantaneous speed, ''average speed'' is defined as the total distance covered over the time interval. For example, if a distance of 80 kilometres is driven in 1 hour, the average speed is 80 kilometres per hour. Likewise, if 320 kilometres are travelled in 4 hours, the average speed is also 80 kilometres per hour. When a distance in kilometres (km) is divided by a time in hours (h), the result is in kilometres per hour (km/h). Average speed does not describe the speed variations that may have taken place during shorter time intervals (as it is the entire distance covered divided by the total time of travel), and so average speed is often quite different from a value of instantaneous speed.<ref name="Hewitt 2006, p. 42"/> If the average speed and the time of travel are known, the distance travelled can be calculated by rearranging the definition to
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| :<math>d = \boldsymbol{\bar{v}}t\,.</math>
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| Using this equation for an average speed of 80 kilometres per hour on a 4-hour trip, the distance covered is found to be 320 kilometres.
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| Expressed in graphical language, the [[slope]] of a [[tangent line]] at any point of a distance-time graph is the instantaneous speed at this point, while the slope of a [[chord (geometry)|chord line]] of the same graph is the average speed during the time interval covered by the chord.
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| ===Tangential speed===
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| Linear speed is the distance traveled per unit of time, while '''tangential speed''' (or tangential velocity) is the linear speed of something moving along a circular path.<ref name="Hewitt 2006, p. 131">Hewitt (2006), p. 131</ref> A point on the outside edge of a [[merry-go-round]] or [[turntable]] travels a greater distance in one complete rotation than a point nearer the center. Travelling a greater distance in the same time means a greater speed, and so linear speed is greater on the outer edge of a rotating object than it is closer to the axis. This speed along a circular path is known as ''tangential speed'' because the direction of motion is [[Tangent lines to circles|tangent]] to the [[circumference]] of the circle. For circular motion, the terms linear speed and tangential speed are used interchangeably, and both use units of m/s, km/h, and others.
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| [[Rotational speed]] (or ''angular speed'') involves the number of revolutions per unit of time. All parts of a rigid merry-go-round or turntable turn about the axis of rotation in the same amount of time. Thus, all parts share the same rate of rotation, or the same number of rotations or revolutions per unit of time. It is common to express rotational rates in revolutions per minute (RPM) or in terms of the number of "radians" turned in a unit of time. There are little more than 6 radians in a full rotation (2{{pi}} radians exactly). When a direction is assigned to rotational speed, it is known as rotational velocity or angular velocity. Rotational velocity is a vector whose magnitude is the rotational speed.
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| Tangential speed and rotational speed are related: the greater the RPMs, the larger the speed in metres per second. Tangential speed is directly proportional to rotational speed at any fixed distance from the axis of rotation.<ref name="Hewitt 2006, p. 131"/> However, tangential speed, unlike rotational speed, depends on radial distance (the distance from the axis). For a platform rotating with a fixed rotational speed, the tangential speed in the centre is zero. Towards the edge of the platform the tangential speed increases proportional to the distance from the axis.<ref>Hewitt (2006), p. 132</ref> In equation form:
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| :<math>v \propto \!\, r \omega\,,</math>
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| where ''v'' is tangential speed and ω (Greek letter [[omega]]) is rotational speed. One moves faster if the rate of rotation increases (a larger value for ω), and one also moves faster if movement farther from the axis occurs (a larger value for ''r''). Move twice as far from the rotational axis at the centre and you move twice as fast. Move out three times as far and you have three times as much tangential speed. In any kind of rotating system, tangential speed depends on how far you are from the axis of rotation.
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| When proper units are used for tangential speed ''v'', rotational speed ω, and radial distance ''r'', the direct proportion of ''v'' to both ''r'' and ω becomes the exact equation
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| :<math>v = r\omega\,.</math>
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| Thus, tangential speed will be directly proportional to ''r'' when all parts of a system simultaneously have the same ω, as for a wheel, disk, or rigid wand. (The direct proportionality of ''v'' to ''r'' is not valid for [[planet]]s, because planets have different rotational speeds).
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| ==Units==
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| {{main|Conversion of units#Speed or velocity}}
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| Units of speed include:
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| *[[metres per second]] (symbol m s<sup>−1</sup> or m/s), the [[SI derived unit]];
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| *[[kilometres per hour]] (symbol km/h);
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| *[[miles per hour]] (symbol mi/h or mph);
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| *[[knot (unit)|knots]] ([[nautical mile]]s per hour, symbol kn or kt);
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| *[[Foot per second|feet per second]] (symbol fps or ft/s);
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| *[[Mach number]] ([[dimensionless]]), speed divided by the [[speed of sound]];
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| *in [[natural units]] (dimensionless), speed divided by the [[speed of light]] in vacuum (symbol ''c'' = {{val|299792458|u=m/s}}).
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| {{Speed conversions}}
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| ==Examples of different speeds==
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| {{Refimprove section|date=May 2013}}
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| {{Main|Orders of magnitude (speed)}}
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| {| class="wikitable"
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| ! Speed !!m/s !!ft/s !!km/h !!mph !!Notes
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| | Approximate rate of [[continental drift]] || {{val|0.00000001}} || {{val|0.00000003}} || {{val|0.00000004}} || {{val|0.00000002}} || 4 cm/year. Varies depending on location.
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| | Speed of a common [[snail]] || 0.001 || 0.003 || 0.004 || 0.002 || 1 millimetre per second
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| | A brisk [[walk]] || 1.7 || 5.5 || 6.1 || 3.8 ||
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| | A typical road cyclist || 4.4 || 14.4 || 16 || 10 || Varies widely by person, terrain, bicycle, effort, weather
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| | A fast martial arts kick || 7.7 || 25.2 || 27.7 || 17.2 || Fastest kick recorded at 130 milliseconds from floor to target at 1 meter distance. Average velocity speed across kick duration<ref name="KickSpeed">http://www.kickspeed.com.au/Improve-measure-kicking-speed.html</ref>
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| | [[Sprint runner]]s || 12.2 || 40 || 43.92 || 27 || Usain Bolt's 100 metre record.
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| | Approximate average speed of road cyclists || 12.5 || 41.0 || 45 || 28 || On flat terrain, will vary
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| | Typical suburban speed limit in most of the world || 13.8 || 45.3 || 50 || 30 ||
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| | [[Taipei 101]] observatory elevator || 16.7 || 54.8 || 60.6 || 37.6 || 1010 m/min
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| | Typical rural speed limit || 24.6 || 80.66 || 88.5 || 56||
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| | British National Speed Limit (single carriageway) || 26.8 || 88 || 96.56 || 60 ||
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| | [[Saffir-Simpson_Hurricane_Scale#Category_1|Category 1]] hurricane || 33 || 108 || 119 || 74 || Minimum sustained speed over 1 minute
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| | Speed limit on a [[Autoroutes of France|French autoroute]] || 36.1 || 118 || 130 || 81 ||
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| | Highest recorded human-powered speed || 37.02 || 121.5 || 133.2 || 82.8 || [[Sam Whittingham]] in a [[recumbent bicycle]]<ref name="sam">http://www.wisil.recumbents.com/wisil/whpsc2009/results.htm</ref>
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| | [[Muzzle velocity]] of a [[paintball marker]] || 90 || 295 || 320 || 200 ||
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| | Cruising speed of a [[Boeing 747-8]] passenger jet || 255 || 836 || 917 || 570 || [[Mach number|Mach]] 0.85 at {{val|35000|u=ft}} ({{val|10668|u=m}}) altitude
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| | The official [[land speed record]] || 341.1 || 1119.1 || 1227.98 || 763 ||
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| | The [[speed of sound]] in dry air at sea-level pressure and 20 °C || 343 || {{val|1125}} || {{val|1235}} || 768 || [[Mach number|Mach]] 1 by definition. 20 °C = 293.15 [[kelvins]].
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| | [[Muzzle velocity]] of an [[AK47]] [[assault rifle]] [[bullet]] || 710 || {{val|2330}} || {{val|2600}} || {{val|1600}} ||
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| | Official [[flight airspeed record]] for jet engined aircraft || 980 || {{val|3215}} || {{val|3530}} || {{val|2194}} || [[Lockheed SR-71 Blackbird]]
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| | [[Space shuttle]] on re-entry || {{val|7800}} || {{val|25600}} || {{val|28000}} || 17,500 ||
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| | [[Escape velocity]] on Earth || {{val|11200}} || {{val|36700}} || {{val|40000}} || {{val|25000}} || 11.2 km·s<sup>-1</sup>
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| |[[Voyager 1]] relative velocity to the Sun in 2013 || {{val|17000}} || {{val|55800}} || {{val|61200}} || {{val|38000}} || Fastest heliocentric [[Recessional velocity|recession speed]] of any humanmade object.<ref>{{cite web |url=http://www.daviddarling.info/encyclopedia/F/fastest_spacecraft.html |title=Fastest Spacecraft |first=David |last=Darling |accessdate=August 19, 2013}}</ref> (11 mi/s)
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| | Average orbital speed of planet [[Earth]] || {{val|29783}} || {{val|97713}} || {{val|107218}} || {{val|66623}} ||
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| | [[Speed of light]] in [[vacuum]] (symbol ''c'') || {{val|299792458}} || {{val|983571056}} || {{val|1079252848}} || {{val|670616629}} || Exactly {{val|299792458|u=m/s}}, by definition of the [[metre]]
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| |}
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| <!-- Could add references for all of these - I think they all come from the linked articles.-->
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| <!-- Removed these two sentences - put back in later? -->
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| <!-- Objects that move horizontally as well as vertically (such as [[aircraft]]) distinguish [[V speeds|forward speed]] and [[V speeds|climbing speed]]. -->
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| Vehicles often have a [[speedometer]] to measure the speed they are moving.
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| ==See also==
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| {{columns-list|2|
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| *[[Air speed]]
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| *[[Land speed]]
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| *[[List of vehicle speed records]]
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| *[[Speedometer]]
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| *[[Projectile#Typical_projectile_speeds|Typical projectile speeds]]
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| *[[V speeds]]}}
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| ==References==
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| {{Wiktionary|speed|swiftness}}
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| {{wikiquote}}
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| * [[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.
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| {{Reflist}}
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| {{Kinematics}}
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| [[Category:Physical quantities]]
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| [[Category:Velocity]]
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| [[de:Geschwindigkeit]]
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| [[ru:Скорость]]
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