Units. Unit Systems. Systems set up fundamental units. British system - foot, pound, second Metric system - meter, kilogram, second. Time. The unit of time originally was based on the day and the year. The second was 1/60 * 1/60 *1/24 of a day.
Prefixes on units are used to represent powers of ten.
Prefixes denote powers of ten from -18 to +18 in steps of three.
Example: a kilometer is 103 meters or 1000 meters.
micro () 10-6
milli (m) 10-3
kilo (k) 103
mega (M) 106
Common, but the power is not a factor of three.
centi (c ) 10-2
deci (d ) 10-1
There are other fundamental units in SI. See NIST.
Derived units are built from the fundamental units.
force (kg m/s2) or (N)
energy (kg m2/s2) or (J)
Salt (solid): 2.165 x 103 kg/m3
Water (liquid): 1.000 x 103 kg/m3
Nitrogen (gas): 1.251 kg/m3