3.

# SI Units

The metric system defines a small set of base units which can be combined to form all other needed units in a second way. For instance, the base unit of length is the meter (m) and the base unit of time is the second (s). The unit for speed is m/s, which is called a derived unit because it is built up of multiple base units.The original metric system defines the gram (g) as the base unit for mass. However, the gram is really too small for day-to-day use, and so most physicists use what is called the MKS (meter-kilogram-second) system of units, also known as SI units. (*SI* stands for *Systeme Internationale* in French).

There are seven base units in the SI system, but we will only use five in this textbook:

- For length, we use the meter (m). For reference, may have seen a meter stick which is a meter long; a yardstick is a little shorter. The distance from the center of your body to your fingertips is about one meter.
- For mass, we use the kilogram (kg). A pineapple or an unabridged dictionary are about 1 kilogram in mass.
- For time, we use the second (s), and I assume you know what that is.
- For temperature, we use the Kelvin (K).
- For electric current, we use the Ampere (A), which is about the size of the electric current that runs in household wires.

*mole*for the amount of substance (which often comes up in chemistry) and the

*candela*(cd) for luminous intensity.

All other units are combinations of these five, although the combinations themselves frequently have names. For example,

- speed is measured in $\u{m/s}$
- force is measured in $\u{kg\cdot m/s^2}$, which is called the
*Newton*(N) - area is measured in $\u{m^2}$
- electric charge is measured in $\u{A\cdot s}$, which is called the
*Coulomb*(C).

The nice thing about the SI system is that, so long as you consistently use SI units and you use equations correctly, then your answer will automatically have the correct SI units.