Speed is a measure. It is velocity but without the direction.[1][2][3][4]
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]
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.
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.
Wikiquote has a collection of quotations related to: Speed
- Orbital speed
- Speedometer – a thing showing speed
- Speed limit – a law telling people a maximum speed
- Speed of sound
- Wind speed
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