- For alternate uses, see speed (disambiguation).
Speed (symbol: v) is the rate of motion, or equivalently the rate of change of position, expressed as distance d moved per unit of time t.
Speed is a scalar quantity with dimensions Length/Time; the equivalent vector quantity to speed is known as velocity. Speed is measured in the same physical units of measurement as velocity, but does not contain the element of direction that velocity has. Speed is thus the magnitude component of velocity.
Units of speed include:
- Mach 1 = 343 m/s (=speed of sound under average circumstances) = 1234.8 km/h
- c = 299,792,458 m/s
- [other important conversions]
- 1 m/s = 3.6 km/h
- 1 mph = 1.609 km/h
- 1 knot = 1.852 km/h = 0.514 m/s
Vehicles often have a speedometer to measure the speed.
The rate of change of speed with respect to time is termed acceleration.