Back

This article needs additional citations for verification. (July 2016) |

Speed | |
---|---|

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.

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 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.

**^**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.**^**Elert, Glenn. "Speed & Velocity".*The Physics Hypertextbook*. Retrieved 8 June 2017.