Units
Always Include Units When Possible
When reporting a measurement, it is important to include units because they serve as a standardized reference. For example, if I tell someone an object traveled "1.5 meters," they will have an understanding of how far that object traveled so long as they know what a meter is. If I do not include units, and I just tell them that the object traveled "1.5," they will not know if I mean 1.5 meters, 1.5 kilometers, or some other unit of distance.
Base Units
Base units serve as fundamental units that are used to measure basic properties. They are arbitrarily defined, meaning scientists have given them a specific value. Unlike derived units (see below), base units are not the result of combining other units together. Below is a table of the base units used by the SI (International System, or Système international (d'unités) in French). SI units are usually preferred by scientists, although there are sometimes exceptions due to convenience. For example, kilograms are the SI unit for mass, but sometimes grams are used for smaller amounts.
Base SI units
*The base unit for mass, the kilogram, has the prefix "kilo." Please see the section about prefixes in the section below.
Derived Units
Units can be combined mathematically, much like algebraic variables. Combining units in this way is helpful for measuring more complicated properties. For example, in order to measure surface area, we can multiple meters by meters to obtain meters squared (m x m = m2). Or to measure volume, we can raise meters to the third power to form m3.
Derived Unit table
Prefixes
Prefixes are used to modify based units when the base unit itself it too small or too large to conveniently report a measurement. For example, while the base unit distance is the meter, but a lager unit would be better to describe distances between cities. Prefixes can increase or decrease a unit by a some factor of 10. The prefix kilo effectively means "multiply the base unit by 1000." So 1 kilometer is 1000 meters and kilograms is 1000 grams.
Common Prefixes Used*
*These are just common prefixes used, but there are more! (Click here for more information.)