In everyday use and in kinematics, the speed of an object is the magnitude of its velocity (the rate of change of its position); it is thus a scalar quantity.  The average speed of an object in an interval of time is the distance travelled by the object divided by the duration of the interval;  the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero.
Speed has the dimensions of distance divided by time. The SI unit of speed is the metre per second, but the most common 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 is commonly used.
The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in a vacuum c = 792 458 metres per second (approximately 299 079 000 000 km/h or 1 000 000 mph ). 671 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.
Italian physicist Galileo Galilei is usually 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. In equation form, this is
where is speed, is distance, and 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).
Speed at some instant, or assumed constant during a very short period of time, is called instantaneous speed . By looking at a speedometer, one can read the instantaneous speed of a car at any instant. 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.
If is the length of the path (also known as the distance) travelled until time , the speed equals the time derivative of : 
In the special case where the velocity is constant (that is, constant speed in a straight line), this can be simplified to . The average speed over a finite time interval is the total distance travelled divided by the time duration.
Different from instantaneous speed, average speed is defined as the total distance covered divided by 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. If the average speed and the time of travel are known, the distance travelled can be calculated by rearranging the definition to
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.
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 line of the same graph is the average speed during the time interval covered by the chord.
Linear speed is the distance travelled per unit of time, while tangential speed (or tangential velocity) is the linear speed of something moving along a circular path. 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 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.
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 π 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.
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. 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. In equation form:
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.
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
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.
Units of speed include:
- metres per second (symbol m s −1 or m/s), the SI derived unit ;
- kilometres per hour (symbol km/h);
- miles per hour (symbol mi/h or mph);
- knots ( nautical miles per hour, symbol kn or kt);
- feet per second (symbol fps or ft/s);
- Mach number ( dimensionless), speed divided by the speed of sound ;
- in natural units (dimensionless), speed divided by the speed of light in vacuum (symbol c = 792 458 m/s ). 299
|1 m/s =||1||3.6||936 2.236||844 1.943||840 3.280|
|1 km/h =||778 0.277||1||371 0.621||957 0.539||344 0.911|
|1 mph =||04 0.447||344 1.609||1||976 0.868||667 1.466|
|1 knot =||444 0.514||1.852||779 1.150||1||810 1.687|
|1 ft/s =||0.3048||28 1.097||818 0.681||484 0.592||1|
(Values in bold face are exact.)
Examples of different speeds
|Approximate rate of continental drift||000 01 0.000||000 03 0.000||000 04 0.000||000 02 0.000||4 cm/year. Varies depending on location.|
|Speed of a common snail||0.001||0.003||0.004||0.002||1 millimetre per second|
|A brisk walk||1.7||5.5||6.1||3.8|
|A typical road cyclist||4.4||14.4||16||10||Varies widely by person, terrain, bicycle, effort, weather|
|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 |
|Sprint runners||12.2||40||43.92||27||Usain Bolt's 100 metres world record.|
|Approximate average speed of road cyclists||12.5||41.0||45||28||On flat terrain, will vary|
|Typical suburban speed limit in most of the world||13.8||45.3||50||30|
|Taipei 101 observatory elevator||16.7||54.8||60.6||37.6||1010 m/min|
|Typical rural speed limit||24.6||80.66||88.5||56|
|British National Speed Limit (single carriageway)||26.8||88||96.56||60|
|Category 1 hurricane||33||108||119||74||Minimum sustained speed over 1 minute|
|Speed limit on a French autoroute||36.1||118||130||81|
|Highest recorded human-powered speed||37.02||121.5||133.2||82.8||Sam Whittingham in a recumbent bicycle |
|Muzzle velocity of a paintball marker||90||295||320||200|
|Cruising speed of a Boeing 747-8 passenger jet||255||836||917||570||Mach 0.85 at 000 ft ( 35 668 m ) altitude 10|
|The official land speed record||341.1||1119.1||1227.98||763|
|The speed of sound in dry air at sea-level pressure and 20 °C||343||1125||1235||768||Mach 1 by definition. 20 °C = 293.15 kelvins.|
|Muzzle velocity of a 7.62x39mm cartridge||710||2330||2600||1600||The 7.62×39mm round is a rifle cartridge of Soviet origin|
|Official flight airspeed record for jet engined aircraft||980||3215||3530||2194||Lockheed SR-71 Blackbird|
|Space shuttle on re-entry||7800||600 25||000 28||17,500|
|Escape velocity on Earth||200 11||700 36||000 40||000 25||11.2 km·s −1|
|Voyager 1 relative velocity to the Sun in 2013||000 17||800 55||200 61||000 38||Fastest heliocentric recession speed of any humanmade object.  (11 mi/s)|
|Average orbital speed of planet Earth around the Sun||783 29||713 97||218 107||623 66|
|The fastest recorded speed of the Helios probes.||70,220||230,381||252,792||157,078||Recognized as the fastest speed achieved by a man-made spacecraft, achieved in solar orbit.|
|Speed of light in vacuum (symbol c )||792 458 299||571 056 983||079 252 848 1||616 629 670||Exactly 792 458 m/s , by definition of the 299 metre|
According to Jean Piaget, the intuition for the notion of speed in humans precedes that of duration, and is based on the notion of outdistancing.  Piaget studied this subject inspired by a question asked to him in 1928 by Albert Einstein: "In what order do children acquire the concepts of time and speed?"  Children's early concept of speed is based on "overtaking", taking only temporal and spatial orders into consideration, specifically: "A moving object is judged to be more rapid than another when at a given moment the first object is behind and a moment or so later ahead of the other object."