 Everipedia is now IQ.wiki - Join the IQ Brainlist and our Discord for early access to editing on the new platform and to participate in the beta testing. # Speed

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 = 299792458 metres per second (approximately 1079000000 km/h or 671000000 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.

Speed
v
SI unitm/s, m s
DimensionLT

## Definition

### Historical definition

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, that is

### Instantaneous speed

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.

### Average speed

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. Average speed of an object is Vav = s÷t

### Difference between speed and velocity

Speed denotes only how fast an object is moving, whereas velocity describes both how fast and in which direction the object is moving. If a car is said to travel at 60 km/h, its speed has been specified. However, if the car is said to move at 60 km/h to the north, its velocity has now been specified.

The big difference can be discerned when considering movement around a circle. When something moves in a circular path and returns to its starting point, its average velocity is zero, but its average speed is found by dividing the circumference of the circle by the time taken to move around the circle. This is because the average velocity is calculated by considering only the displacement between the starting and end points, whereas the average speed considers only the total distance travelled.

### Tangential speed

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 (2π 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

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 = 299792458 m/s).

Conversions between common units of speed
m/skm/hmphknotft/s
1m/s=13.62.2369361.9438443.280840
1km/h=0.27777810.6213710.5399570.911344
1mph=**0.44704****1.609344**10.8689761.466667
1knot=0.5144441.8521.15077911.687810
1ft/s=**0.3048****1.09728**0.6818180.5924841

(Values in bold face are exact.)

## Examples of different speeds

Speedm/sft/skm/hmphNotes
Approximate rate ofcontinental drift0.000000010.000000030.000000040.000000024 cm/year.Varies depending on location.
Speed of a commonsnail0.0010.0030.0040.0021 millimetre per second
A briskwalk1.75.56.13.8
A typical road cyclist4.414.41610Varies widely by person, terrain, bicycle, effort, weather
A fast martial arts kick7.725.227.717.2Fastest kick recorded at 130 milliseconds from floor to target at 1 meter distance.Average velocity speed across kick duration
Sprint runners12.24043.9227Usain Bolt's100 metresworld record.
Approximate average speed of road cyclists12.541.04528On flat terrain, will vary
Typical suburban speed limit in most of the world13.845.35030
Taipei 101observatory elevator16.754.860.637.61010 m/min
Typical rural speed limit24.680.6688.556
British National Speed Limit (single carriageway)26.88896.5660
Category 1hurricane3310811974Minimum sustained speed over 1 minute
Speed limit on aFrench autoroute36.111813081
Highest recorded human-powered speed37.02121.5133.282.8Sam Whittinghamin arecumbent bicycle
Muzzle velocityof apaintball marker90295320200
Cruising speed of aBoeing 747-8passenger jet255836917570Mach0.85 at35000ft(10668m) altitude
The officialland speed record341.11119.11227.98763
Thespeed of soundin dry air at sea-level pressure and 20 °C34311251235768Mach1 by definition.20 °C = 293.15kelvins.
Muzzle velocityof a7.62×39mmcartridge710233026001600The7.62×39mmround is a riflecartridgeof Sovietorigin
Officialflight airspeed recordfor jet engined aircraft980321535302194Lockheed SR-71 Blackbird
Space shuttleon re-entry7800256002800017,500
Escape velocityon Earth1120036700400002500011.2 km·s−1
Voyager 1relative velocity to the Sun in 201317000558006120038000Fastest heliocentricrecession speedof any humanmade object.(11 mi/s)
Average orbital speed of planetEartharound the Sun297839771310721866623
The fastest recorded speed ofthe Helios probes.70,220230,381252,792157,078Recognized as the fastest speed achieved by a man-made spacecraft, achieved insolar orbit.
Speed of lightin vacuum(symbol c)2997924589835710561079252848670616629Exactly299792458m/s, by definition of themetre

## Psychology

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."

• Air speed

• Land speed

• List of vehicle speed records

• Typical projectile speeds

• Speedometer

• V speeds

## References


Citation Linkhdl.handle.netWilson, Edwin Bidwell (1901). Vector analysis: a text-book for the use of students of mathematics and physics, founded upon the lectures of J. Willard Gibbs. p. 125. This is the likely origin of the speed/velocity terminology in vector physics.
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Citation Linkwww.electropedia.org"IEC 60050 - Details for IEV number 113-01-33: "speed"". Electropedia: The World's Online Electrotechnical Vocabulary. Retrieved 2017-06-08.
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Citation Linkwww.daviddarling.infoDarling, David. "Fastest Spacecraft". Retrieved August 19, 2013.
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Citation Linkopenlibrary.orgJean Piaget, Psychology and Epistemology: Towards a Theory of Knowledge, The Viking Press, pp. 82–83 and pp. 110–112, 1973. SBN 670-00362-x
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Citation Link//doi.org/10.1037%2F0012-1649.15.3.288Siegler, Robert S.; Richards, D. Dean (1979). "Development of Time, Speed, and Distance Concepts" (PDF). Developmental Psychology. 15 (3): 288–298. doi:10.1037/0012-1649.15.3.288.
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Citation Linkbooks.google.comRod Parker-Rees and Jenny William, eds. (2006). Early Years Education: Histories and Traditions, Volume 1. Taylor & Francis. p. 164.CS1 maint: uses editors parameter (link)
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