# Speed

In kinematics, the speed of an object is the magnitude of its velocity (the rate of change of its position); it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero.

Like velocity, speed has the dimensions of a length divided by a time; the SI unit of speed is the meter per second, but the most usual unit of speed in everyday usage is the kilometer per hour or, in the USA and the UK, miles per hour. 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 = 299,792,458 meters per second, approximately 1079 million kilometers per hour (671,000,000 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. In day-to-day athletics, it is proper to say that a teenager can achieve at least 20 km/h (or 12.43 mph) of speed while a best runner can achieve 30 km/h (or 18.64 mph) which is similar to running 100 metres in about 12 seconds. The average speed for a teenager is 24 km/h, which can be a result of running 100 m in 15 seconds.

### Famous quotes containing the word speed:

If it be aught toward the general good,
Set honor in one eye, and death i’th’ other,
And I will look on both indifferently;
For let the gods so speed me as I love
The name of honor more than I fear death.
William Shakespeare (1564–1616)

Spig Wead: I’ve been thinking what a heel I’ve been about you and about my own kids. I don’t know, when I do something, I go all the way. Living. Gambling. Flying. I tap myself out. I guess that’s the way I want it to be. Maybe even the way I am.
Minne Wead: Star-spangled Spig. Damn the martinis, full speed ahead and don’t give up the ship.
Frank Fenton, William Wister Haines, co-scenarist, and John Ford. Spig Wead (John Wayne)

There was such speed in her little body,
And such lightness in her footfall,
It is no wonder her brown study
Astonishes us all.
John Crowe Ransom (1888–1974)