Description: To describe linear motion, you need to know the distance between a start and end position. The speed is the distance between positions divided by the time it takes to move between them.
Key Terms: Position, distance, meter, speed, time, divide

Before the time of Galileo, people described moving things as simply “slow” or “fast.” Such descriptions were vague. Galileo is credited with being the first to measure speed by considering the distance covered and the time it takes. He defined speed as the distance covered per unit of time. Interestingly, Galileo could easily measure distance, but in his day measuring short times was no easy matter. He sometimes used his own pulse and sometimes the dripping of drops from a “water clock” he devised.

A cyclist who covers 16 meters in a time of 2 seconds, for example, has a speed of 8 meters per second. Any combination of distance and time units is legitimate for measuring speed; for motor vehicles (or long distances), the units kilometers per hour (km/h) or miles per hour (mi/h or mph) are commonly used. For shorter distances, meters per second (m/s) is more useful. The slash symbol (/) is read as per and means “divided by.”