# Speed

magnitude of velocity of motion

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:

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.