Editing Speed (section)

{{Short description|Magnitude of velocity}}
{{About|the property of moving bodies|other uses|Speed (disambiguation)}}
{{pp-semi-indef|small=yes}}
{{more citations needed|date=July 2016}}
{{Use Oxford spelling|date=September 2019}}
{{Infobox physical quantity
| name = Speed
| image = Motorcyclist in Midtown Manhattan-L1002704.jpg
| image_size = 250px
| 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.
| unit = m/s, m&nbsp;s<sup>−1</sup>
| symbols = ''v''
| dimension = '''L''' '''T'''<sup>−1</sup>
}}
In [[kinematics]], the '''speed''' (commonly referred to as '''''v''''') of an object is the [[magnitude (mathematics)|magnitude]] of the change of its [[Position (vector)|position]] over time or the magnitude of the change of its position per unit of time; it is thus a non-negative [[Scalar (physics)|scalar]] quantity.<ref name="HSM2023">{{cite web|title=Origin of the speed/velocity terminology|publisher=History of Science and Mathematics Stack Exchange |url=https://hsm.stackexchange.com/questions/10984/why-are-speed-and-velocity-not-given-the-same-name/|access-date=12 June 2023}} Introduction of the speed/velocity terminology by Prof. Tait, in 1882.</ref> The '''average speed''' of an object in an interval of time is the [[distance]] travelled by the object divided by the [[Time in physics|duration]] of the interval;<ref name=":0">{{cite web|url=http://physics.info/velocity/|title=Speed & Velocity|last=Elert|first=Glenn|website=The Physics Hypertextbook|access-date=8 June 2017}}</ref> the '''instantaneous speed''' is the [[limit (mathematics)|limit]] of the average speed as the duration of the time interval approaches zero. Speed is the magnitude of [[#Difference between speed and velocity|''velocity'']] (a vector), which indicates additionally the direction of motion.

Speed has the [[dimensional analysis|dimension]]s of distance divided by time. The [[International System of Units|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 (unit)|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 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.