Units are the references we use to measure real world phenomena. They are either defined in relation to real world measurement, or in relation to other units.
Units themselves do not carry quantities, nor do quantities carry units. That said, both must be notated for concrete measurements. Twelve is a an abstract quantity, and egg is an abstraction of a physical item. A dozen eggs, on the other hand, is something you can buy at the grocery store.
Quantities are often rounded to an extent. Three decimal points or three digits of precision is usually a good rule of thumb where precision is not required. Excessive precision makes you look like a know-it-all, or a conspiracy theorist.
Examples of notation for the frequency 20,327 Hz:
Quantities are often specified by common prefixes.
Scientific notation specifies quantity in the format of a × 10n for 1 ≤ a < 10 and any integer n.
*10^n can be replaced by
Engineering notation is the same as scientific notation, but with exponents
only divisible by 3. This makes them align with prefixes and makes verbal
communication easier. Re-using the above example, 48,000 would be written as
SI base units
Most base units have a historical definition grounded in simpler measurements. In modern times, they are put in relation to very specific physical constants in order to increase precision and to account for modern scientific theories.
For more, see Wikipedia: International System of Units.
Time has the common symbol
|second||s||Historically, a division of the day|
Length has the common symbol
|meter||m||Historically, 10,000 km was the distance from the equator to the north pole|
|inch||in or “||2.54 cm|
|foot||foot or ‘||12 in|
Mass has the common symbol
m. It is commonly referred to as weight, but
weight is rather force caused by gravity.
|gram||g||Historically, 1 kg is the mass of one liter of water.|
|pound||lb||circa 0.454 kg|
Temperature has the common symbol
|Kelvin||K||The same scale as celsius, but 0K, absolute zero, equals -273.15°C|
|Celsius||°C||Linear scale where 0°C is the freezing temperature of water and 100°C is the boiling temperature of water|
|Farenheit||°F||Exact origins vague, but temperature defined in relation to Celsius as
|Rankine||°Ra||As Kelvin is to Celsius, T°Ra = 5/9 × TK|
Current uses the symbol
|Ampere||A||Historically, coloumb per second|
The SI definition of ampere was formerly charge over time, with ampere being defined as coloumb per second. Later revisions has reversed the relation, and an ampere is technically defined directly as 6.241509074×1018 electrons per second.
Unnamed SI derived units
A few units relatively fundamental to the human experience don’t have SI names and are just referred to by their relation to the base units. However, historical named units may exist.
meter per second knop
meter per second per second
square meter hectare american football fields
cubic meter liter ounces
Named SI derived units
Voltage (sv: spänning, literally “tension”) measures electrical potential of charge.