In physics class, you will be using the metric system.  In fact, almost every science class you take will use the metric system.

Officially, what we call the metric system is called the International System of Units.  It is often shortened to the SI system, because of the French order of words, Systeme Internationale d’unites.  This system uses seven base units for time, length, mass, temperature, brightness, current, and amount of substance.

The Metric System

The “metric” part of the name comes from the idea that most measurements and sub-measurements in this system are based on powers of ten.  For example, there are 100 centimeters in a meter.  1000 meters make up a kilometer.  This system allows us to “simply” move the decimal point in a measurement to change the size of the unit.  This is in contrast to the Imperial system, in which we have strange things like 12 inches in a foot and 5280 feet in a mile.  Quick, how many teaspoons in a cup?

The Base Units

  • Length in meters:  This length was originally based on the distance between the north pole and the equator.
  • Time in seconds: Originally a fraction of a day.
  • Mass in kilograms: Based on the mass of a particular object located in a vault in Paris.
  • Temperature in Kelvin:  Kelvin temperature relates absolute zero and the triple point of water.
  • Brightness in candela: Based on the brightness of a standard light source
  • Current in amperes:  The current required to provide a particular force between two parallel wires
  • Amount of substance in moles: The number of elementary particles (atoms or molecules) in 12 grams of carbon 12

In the first semester of physics, we usually focuses on quantities that can be measured using meters, kilograms, and seconds (the MKS system).  During the second semester, we will focus on temperature, brightness, current, and measurements in moles.

(More information about the particular measurements for the base units can be found on the International System of Units page in wikipedia.

Derived Units

We refer to all other measurements in physics as derived units.   While this might sound difficult, in reality, you have been using derived units often.  A derived unit is simply a combination that includes two or more of the base units.  For example, we measure speed in m/s or meters per second.  Later in the semester, we will measure forces in newtons, a combination of kg m/s².  You will learn many of these derived units as we go.  I recommend that you memorize any new derived units as you learn about them.  Knowing the combinations will help you understand the measurements better.  You can often find and fix your mistakes when you know how to check your units.