Science is based upon experimental measurements, and measurements require units. The system of units used in science is called le Système Internationale d’Unités. These are commonly referred to as SI units. In casual speaking we often refer to metric units.

All of the quantities needed to understand motion can be expressed in terms of the three basic SI units shown in TABLE 1.2. Other quantities can be expressed as a combination of these basic units. Velocity, expressed in meters per second or $\text{m}\text{/}\text{s}$, is a ratio of the length unit to the time unit.