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Speed

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.

<math>

v={d \over t} <math>

Units of speed include:

• metre per second, (symbol m/s), the SI derived unit
• kilometres per hour, (symbol km/h)
• miles per hour, (symbol mph)
• knot (nautical miles per hour)
• Mach, where Mach 1 is the speed of sound; Mach n is n times as fast.
• speed of light in vacuum (symbol c) is one of the natural units

1 m/s = 3.6 km/h

1 mph = 1.609 km/h

1 knot = 1.852 km/h = 0.514 m/s

Mach 1 = 343 m/s (=speed of sound under average circumstances) = 1234.8 km/h

c = 299,792,458 m/s

Vehicles often have a speedometer to measure the speed.

The rate of change of speed with respect to time is termed acceleration. Because speed has no direction, any acceleration derived from a speed will be a scalar quantity or magnitude, and will also have no direction.