# Speed

magnitude of velocity of motion

Speed is a measure. It is velocity but without the direction.

## Finding speed

Speed is the distance an object moves in a given amount of time. The distance is never negative. 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. During this one hour, the train may become slower and faster, it may even drive backwards. The average speed of an object in a certain time is the distance the object traveled divided by the time. The instantaneous speed is the average speed when the time is very small, almost zero.

## 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). 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. You can write bigger speeds, but they are not possible in this universe.