Introduction

The SI (Système international d'unités) system is the standard system of measurement used in science and engineering. It is a variation on the metric system, which is famous for not being used in the United States. The SI system is based on a set of base units, which are defined in terms of fundamental physical constants. Some of the most common base units are the meter ($\textrm{m}$) for distance, kilogram ($\textrm{kg}$) for mass, and second ($\textrm{s}$) for time.

As you progress this course, you will learn about more SI units that are used to measure other physical quantities, such as the Newton
($\textrm{N}$) for force, the Joule ($\textrm{J}$) for energy, etc. But what if we want to measure the distance, say, from the Earth to the sun? Or the mass of a proton? These values are so large or small that they are impractical to express in base units. This is why the SI system also includes a set of metric prefixes, which are used to express these values in a more manageable way. These prefixes are used to multiply or divide the base unit by a power of ten, allowing us to express very large or very small quantities in a more convenient form.

One of the base SI units is actually expressed in terms of a metric prefix: the kilogram ($\textrm{kg}$) is defined as 1000 grams ($\textrm{g}$), where the prefix "kilo-" means "1000". This is just one example of how metric prefixes are used to express quantities in the SI system. The kilogram is sort of unique in that it is the only base SI unit that has a metric prefix in its name, and that's because the gram is a very small unit of mass.

The metric prefixes are expressed as an extra part of the unit, such as "kilo" in "kilogram" or "centi" in "centimeter". In the abbreviations of the unit, they are usually expressed with a one or two letter abbreviation that is placed before the base unit, such as "k" for kilo (i.e. $\textrm{kg}$) or "c" for centi (i.e. $\textrm{cm}$). Here is a list of all metric prefixes used in the SI system:

Memorize all of these. It's absolutely essential to know them, and you should sacrifice some time to do so. It should only take two or three hours of constant and agonizing repetition.

I am joking. You definitely don't need all of these. After all, who's ever used the prefix "yotta-" in their life? There are a few common prefixes that you need to memorize, and that'll be it. Those are:

Isn't that just so much more manageable? You can definitely memorize this. Even within this list, the prefixes "kilo", "centi", and "milli" are seen far more often than the others. These prefixes are the most commonly used in physics, and you will see them in the future.

Now that you know the metric prefixes, let's practice using them! Below is a quick question that will test your understanding of the common metric prefixes. Try to answer the question without looking at the table!