Units of Measurement

System International (SI) units

Length: based on the meter (length from your nose to your hand).

1 kilometer = 1 km = 10³ m.
1 meter = 1 m.
1 centimeter = 1 cm = 10^-2 m.
1 millimeter = 1 mm = 10^-3 m.
1 micrometer = 10^-6 m. (abbreviation: Greek mu + m)
1 nanometer = 1 nm = 10^-9 m.

Time: based on the second ("one thousand one ...").

Temperature: based on the Kelvin

0 K is "absolute zero."
water freezes at 273 K.
300 K is room temperature.
water boils at 373 K.
cakes bake at 450 K.

Frequency:

Frequency is number of complete waves passing per unit time. It is measured in Hertz (Hz), the number per second.

1 KHz = 10³ Hz.
1 MHz = 10⁶ Hz.
For sound, "A" is 440 Hz.

Energy and Power:

Energy is measured in Joules, J. The rate at which energy is transfered ("power") is measured in Joules per second. One Joule per second is called a Watt, W.

a lightbulb typically consumes 100 W of power.
a typical person consumes about 1000 W = 1 KW for heating, cooking, etc .
The amount of energy received at the Earth per unit time per unit area from the Sun is about 1 KW/m².

Non-System-International (SI) units

Energy:

The electron volt eV is commonly used to measure energy of atoms and molecules, or of individual photons. Of course, an electron volt is a ridiculously small unit for measuring macroscopic amounts of energy.

Angles:

A circle is divided into 360 degrees.
A degree is divided into 60 arc minutes.
An arc minute is divided into 60 arc seconds.

Distance:

The Earth-Sun distance is the astronomical unit (AU): 1 AU = 150 x 10⁶ km.
Astronomers like to use parsecs (pc): 1 pc = 3.09 x 10¹³ km.
Sometimes you see distances in light-years (ly)
- 1 ly = the distance light travels in one year.
- Light travels at a speed c = 3 x 10⁸ m/s.
- One year is about 3 x 10⁷ s.
- So 1 ly = 3 x 10⁸ m/s x 3 x 10⁷ s ~ 10¹⁶ m.
- The relation to parsecs is 1 ly ~ 0.3 pc.

Updated 26 September 2007

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA