# 1.02 A system of units

Most scientist use the SI units. This is an international system of units for each physical quantity. In was implied to make the meaning of numbers universal.

The SI units you need to know for now are

- Mass (m) has the SI unit kilogram (kg)
- Length (x) has the SI unit metre (m)
- Time (t) has the SI unit second (s)

Learn the following prefixes by heart. They can be put in front of any SI unit

SI unit of mass

SI unit of length