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 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
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.
- A circle is divided into 360 degrees.
- A degree is divided into 60 arc minutes.
- An arc minute is divided into 60 arc seconds.
- 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.
60 x 60 x 24 x 365.25 = 31557600.
- So 1 ly = 3 x 108 m/s x 3 x 107 s ~
- 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