Speed

For other uses, see Speed (disambiguation).

Speed is a measure. It is velocity but without the direction.^[1][2][3][4]

Finding speed

Speed is the distance an object moves in a given amount of time.^[1][2][3] The distance is never negative.^[5] If a train takes 1 hour to travel 100 kilometers, it has a speed of 100 kilometers per hour (62 mph). In fact this is the average speed.^[3] During this one hour, the train may become slower and faster, it may even drive backwards.^[3] The average speed of an object in a certain time is the distance the object traveled divided by the time.^[1][3][5] The instantaneous speed is the average speed when the time is very small, almost zero.^[3]

Units

There are many units of measurement. Since the 20th century following units were widely used by humans:

• miles per hour (mph),
• kilometers per hour (km/h), and
• meters per second (m/s), which is the SI-unit for speed.

Different units are used for different applications. People controlling planes and ships frequently use Knot (speed).^[3] Sometimes a Mach number is used.

Range

The smallest speed is 0 meters per second (0 km/h; 0 mph). A “negative speed” would be in fact a velocity. The biggest speed is the speed of light.^[6] You can write bigger speeds, but they are not possible in this universe.

Read also

• Orbital speed
• Speedometer – a thing showing speed
• Speed limit – a law telling people a maximum speed
• Speed of sound
• Wind speed