Speed

Magnitude of velocity

This article is about the property of moving bodies. For other uses, see Speed (disambiguation).

"Slow" and "Slowness" redirect here. For other uses, see Slow (disambiguation).

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
Find sources: "Speed" – news · newspapers · books · scholar · JSTOR (July 2016) (Learn how and when to remove this template message)

|

Speed

File:Mersan.JPG

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.

Common symbols

| v
SI unit| m/s, m s−1
Dimension| L T−1

In everyday use and in kinematics, the speed (commonly referred to as v) of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a scalar quantity.[1] 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;[2] the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero. Speed is not the same as velocity.

Speed has the dimensions of distance divided by time. The SI unit of speed is the metre per second (m/s), but the most common unit of speed in everyday usage is the kilometre per hour (km/h) or, in the US and the UK, miles per hour (mph). 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.

## Contents

* 1 Definition
* 1.1 Historical definition
* 1.2 Instantaneous speed
* 1.3 Average speed
* 1.4 Difference between speed and velocity
* 1.5 Tangential speed
* 2 Units
* 3 Examples of different speeds
* 4 Psychology
* 5 See also
* 6 References

## Definition[edit | edit source]

### Historical definition[edit | edit source]

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.[3] In equation form, that is

v = d t , {\displaystyle v={\frac {d}{t}},}

where  v {\displaystyle v} is speed,  d {\displaystyle d} is distance, and  t {\displaystyle 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).

### Instantaneous speed[edit | edit source]

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.[3] 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.

In mathematical terms, the instantaneous speed  v {\displaystyle v} is defined as the magnitude of the instantaneous velocity v {\displaystyle {\boldsymbol {v}}} , that is, the derivative of the position  r {\displaystyle {\boldsymbol {r}}} with respect to time:[2][4]

v = | v | = | r ˙ | = | d r d t | . {\displaystyle v=\left|{\boldsymbol {v}}\right|=\left|{\dot {\boldsymbol {r}}}\right|=\left|{\frac {d{\boldsymbol {r}}}{dt}}\right|\,.}

If  s {\displaystyle s} is the length of the path (also known as the distance) travelled until time  t {\displaystyle t} , the speed equals the time derivative of  s {\displaystyle s} :[2]

v = d s d t . {\displaystyle v={\frac {ds}{dt}}.}

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 {\displaystyle v=s/t} . The average speed over a finite time interval is the total distance travelled divided by the time duration.

### Average speed[edit | edit source]

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.[3] If the average speed and the time of travel are known, the distance travelled can be calculated by rearranging the definition to

d = v ¯ t . {\displaystyle d={\boldsymbol {\bar {v}}}t\,.}

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[edit | edit source]

Speed denotes only how fast an object is moving, whereas velocity describes both how fast and in which direction the object is moving.[5] 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[edit | edit source]

Part of a series on

Classical mechanics
F = d d t ( m v ) {\displaystyle {\textbf {F}}={\frac {\mathrm {d} }{\mathrm {d} t}}(m{\textbf {v}})}

Second law of motion

* History
* Timeline
* Textbooks

Branches

* Applied
* Celestial
* Continuum
* Dynamics
* Kinematics
* Kinetics
* Statics
* Statistical

Fundamentals

* Acceleration
* Angular momentum
* Couple
* D'Alembert's principle
* Energy
* kinetic
* potential
* Force
* Frame of reference
* Inertial frame of reference
* Impulse
* Inertia / Moment of inertia
* Mass

Mechanical power
* Mechanical work

Moment
* Momentum
* Space
* Speed
* Time
* Torque
* Velocity
* Virtual work

Formulations

* Newton's laws of motion

* Analytical mechanics

* Lagrangian mechanics
* Hamiltonian mechanics
* Routhian mechanics
* Hamilton–Jacobi equation
* Appell's equation of motion
* Koopman–von Neumann mechanics

Core topics

* Damping ratio
* Displacement
* Equations of motion
* Euler's laws of motion
* Fictitious force
* Friction
* Harmonic oscillator

* Inertial / Non-inertial reference frame
* Mechanics of planar particle motion

* Motion (linear)
* Newton's law of universal gravitation
* Newton's laws of motion
* Relative velocity
* Rigid body
* dynamics
* Euler's equations
* Simple harmonic motion
* Vibration

Rotation

* Circular motion
* Rotating reference frame
* Centripetal force
* Centrifugal force
* reactive
* Coriolis force
* Pendulum
* Tangential speed
* Rotational speed

* Angular acceleration / displacement / frequency / velocity

Scientists

* Kepler
* Galileo
* Huygens
* Newton
* Horrocks
* Halley
* Maupertuis
* Daniel Bernoulli
* Johann Bernoulli
* Euler
* d'Alembert
* Clairaut
* Lagrange
* Laplace
* Hamilton
* Poisson
* Cauchy
* Routh
* Liouville
* Appell
* Gibbs
* Koopman
* von Neumann

*   * Category

* v
* t
* e

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.[6] 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.[6] 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.[7] In equation form:

v ∝ r ω , {\displaystyle v\propto \\!\,r\omega \,,}

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

v = r ω . {\displaystyle v=r\omega \,.}

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[edit | edit source]

Main article: Conversion of units § Speed or velocity

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/s | km/h | mph | knot | ft/s

|
|
|
|
|

1 m/s =  | 1 | 3.600000 | 2.236936* | 1.943844* | 3.280840*
1 km/h =  | 0.277778* | 1 | 0.621371* | 0.539957* | 0.911344*
1 mph =  | 0.44704 | 1.609344 | 1 | 0.868976* | 1.466667*
1 knot =  | 0.514444* | 1.852 | 1.150779* | 1 | 1.687810*
1 ft/s =  | 0.3048 | 1.09728 | 0.681818* | 0.592484* | 1

(* = approximate values)

## Examples of different speeds[edit | edit source]

This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (May 2013) (Learn how and when to remove this template message)

|

This section may contain indiscriminate, excessive, or irrelevant examples. Please improve the article by adding more descriptive text and removing less pertinent examples. See Justapedia's guide to writing better articles for further suggestions. (July 2023)

|

Main article: Orders of magnitude (speed)

Speed | m/s | ft/s | km/h | mph | Notes

|
|
|
|
|

Global average sea level rise | 0.00000000011 | 0.00000000036 | 0.0000000004 | 0.00000000025 | 3.5 mm/year[8]
Approximate rate of continental drift | 0.0000000013 | 0.0000000042 | 0.0000000045 | 0.0000000028 | 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[9]
Sprint runners | 12.2 | 40 | 43.92 | 27 | Usain Bolt's 100 metres world record.
Approximate average speed of road race 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
Average peak speed of a cheetah | 33.53 | 110 | 120.7 | 75 |
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[10]
Average speed of Human sneeze | 44.44 | 145.82 | 160 | 99.42 |
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 35000 ft (10668 m) altitude
Speed of a .22 caliber Long Rifle bullet | 326.14 | 1070 | 1174.09 | 729.55 |
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 7.62×39mm 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 | 25600 | 28000 | 17,500 |
Escape velocity on Earth | 11200 | 36700 | 40000 | 25000 | 11.2 km·s−1
Voyager 1 relative velocity to the Sun in 2013 | 17000 | 55800 | 61200 | 38000 | Fastest heliocentric recession speed of any humanmade object.[11] (11 mi/s)
Average orbital speed of planet Earth around the Sun | 29783 | 97713 | 107218 | 66623 |
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.
Orbital speed of the Sun relative to the center of the galaxy | 251000 | 823000 | 904000 | 561000
Speed of the Galaxy relative to the CMB | 550000 | 1800000 | 2000000 | 1240000
Speed of light in vacuum (symbol c) | 299792458 | 983571056 | 1079252848 | 670616629 | Exactly 299792458 m/s, by definition of the metre

## Psychology[edit | edit source]

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.[12] 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?"[13] 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."[14]

## See also[edit | edit source]

* Air speed
* Land speed
* List of vehicle speed records
* Typical projectile speeds
* Speedometer
* V speeds

## References[edit | edit source]

Look up speed or swiftness in Wiktionary, the free dictionary.

Wikiquote has quotations related to Speed.

Wikimedia Commons has media related to Speed.

* 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.

1. ^ Wilson, 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. Yale bicentennial publications. C. Scribner's Sons. p. 125\. hdl:2027/mdp.39015000962285. This is the likely origin of the speed/velocity terminology in vector physics.
2. ^ a b c Elert, Glenn. "Speed & Velocity". The Physics Hypertextbook. Retrieved 8 June 2017.
3. ^ a b c Hewitt (2006), p. 42
4. ^ "IEC 60050 - Details for IEV number 113-01-33: "speed"". Electropedia: The World's Online Electrotechnical Vocabulary. Retrieved 2017-06-08.
5. ^ Wilson, 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. Yale bicentennial publications. C. Scribner's Sons. p. 125\. hdl:2027/mdp.39015000962285. This is the likely origin of the speed/velocity terminology in vector physics.
6. ^ a b Hewitt (2006), p. 131
7. ^ Hewitt (2006), p. 132
8. ^ NASA's Goddard Space Flight Center. "Satellite sea level observations". Global Climate Change. NASA. Retrieved 20 April 2022.
9. ^ "Improve Kicking Speed for Martial Arts | Get Fast Kicks!". Archived from the original on 2013-11-11. Retrieved 2013-08-14.
10. ^ "The Recumbent Bicycle and Human Powered Vehicle Information Center". Archived from the original on 2013-08-11. Retrieved 2013-10-12.
11. ^ Darling, David. "Fastest Spacecraft". Retrieved August 19, 2013.
12. ^ Jean Piaget, Psychology and Epistemology: Towards a Theory of Knowledge, The Viking Press, pp. 82–83 and pp. 110–112, 1973. SBN 670-00362-x
13. ^ Siegler, 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.
14. ^ Early Years Education: Histories and Traditions, Volume 1. Taylor & Francis. 2006. p. 164\. ISBN 9780415326704.

* v
* t
* e

Kinematics

* ← Integrate … Differentiate →

* Absement
* Displacement (Distance)
* Velocity (Speed)
* Acceleration
* Jerk
* Higher derivatives

* v
* t
* e

Classical mechanics SI units

| Linear/translational quantities |  | Angular/rotational quantities

|
|

Dimensions | 1 | L | L2 | Dimensions | 1 | θ | θ2
T | time: t
s | absement: A
m s |  | T | time: t
s |  |
1 |  | distance: d, position: r, s, x, displacement
m | area: A
m2 | 1 |  | angle: θ, angular displacement: θ
rad | solid angle: Ω
rad2, sr
T−1 | frequency: f
s−1, Hz | speed: v, velocity: v
m s−1 | kinematic viscosity: ν,
specific angular momentum: h
m2 s−1 | T−1 | frequency: f
s−1, Hz | angular speed: ω, angular velocity: ω
rad s−1 |
T−2 |  | acceleration: a
m s−2 |  | T−2 |  | angular acceleration: α
rad s−2 |
T−3 |  | jerk: j
m s−3 |  | T−3 |  | angular jerk: ζ
rad s−3 |
M | mass: m
kg | weighted position: M ⟨x⟩ = ∑ m x  |  | ML2 | moment of inertia: I
kg m2 |  |
MT−1 |  | momentum: p, impulse: J
kg m s−1, N s | action: 𝒮, actergy: ℵ
kg m2 s−1, J s | ML2T−1 |  | angular momentum: L, angular impulse: ΔL
kg m2 s−1 | action: 𝒮, actergy: ℵ
kg m2 s−1, J s
MT−2 |  | force: F, weight: Fg
kg  m s−2, N | energy: E, work: W, Lagrangian: L
kg m2 s−2, J | ML2T−2 |  | torque: τ, moment: M
kg m2 s−2, N m | energy: E, work: W, Lagrangian: L
kg m2 s−2, J
MT−3 |  | yank: Y
kg m s−3, N s−1 | power: P
kg m2 s−3, W | ML2T−3 |  | rotatum: P
kg m2 s−3, N m s−1 | power: P
kg m2 s−3, W
*[v]: View this template
*[t]: Discuss this template
*[e]: Edit this template