# Chapter 3: Measurement

## 3.3 The SI system

The metric system is used by every country in the world except for Liberia, Myanmar, and the United States of America. In the metric system, you have a base unit that tells you what kind of quantity is being measured and a prefix that tells you the scale. For example, a meter is a measurement of distance, and the prefix kilo means thousand; so a kilometer (kilometer) is one thousand meters. The following table gives the common metric prefixes that we will be using in this book:

The scientific community uses a particular variety of the metric system called the Système International d’Unités, or SI. The difference between SI and other varieties of the metric system lies in how the base units are defined. The base units in the SI system are the meter for length, the kilogram for mass, and the second for time (it is sometimes called the mks system). This is summarized, with examples, in the following table. This is a very elegant system: whenever we see a prefix in front of a base unit, we automatically know the scale we’re dealing with, and the physical quantity being measured. By convention, the prefix centi is generally only used in length measurements with SI units.