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However, the names of all SI mass units are based on gram, rather than on kilogram; thus 10 3 kg is a megagram (10 6 g), not a *kilokilogram. The tonne (t) is an SI-compatible unit of mass equal to a megagram (Mg), or 10 3 kg. The unit is in common use for masses above about 10 3 kg and is often used with SI prefixes.
A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic.Each prefix has a unique symbol that is prepended to any unit symbol.
The micrometre (SI symbol: μm) is a unit of length in the metric system equal to 10 −6 metres ( 1 / 1 000 000 m = 0. 000 001 m). To help compare different orders of magnitude , this section lists some items with lengths between 10 −6 and 10 −5 m (between 1 and 10 micrometers , or μm).
So are 1 and 2, 1 and 9, or 1 and 0.2. However, 1 and 15 are not within an order of magnitude, since their ratio is 15/1 = 15 > 10. The reciprocal ratio, 1/15, is less than 0.1, so the same result is obtained. Differences in order of magnitude can be measured on a base-10 logarithmic scale in "decades" (i.e., factors of ten). [2]
The barn (b) is a unit of area used in nuclear physics equal to one hundred femtometres squared (100 fm 2 = 10 −28 m 2). The are (a) is a unit of area equal to 100 m 2. The decare (daa) is a unit of area equal to 1000 m 2. The hectare (ha) is a unit of area equal to 10 000 m 2 (0.01 km 2).
In unit systems where force is a derived unit, like in SI units, g c is equal to 1. In unit systems where force is a primary unit, like in imperial and US customary measurement systems, g c may or may not equal 1 depending on the units used, and value other than 1 may be required to obtain correct results. [2]
Unit prefixes that are much larger or smaller than encountered in practice are seldom used, albeit valid combinations. In most contexts only a few, the most common, combinations are established. For example, prefixes for multiples greater than one thousand are rarely applied to the gram or metre.
Gamma (symbol: γ) is a deprecated non-SI unit of mass equal to 1 μg. [2] A fullwidth version of the "microgram" symbol is encoded by Unicode at code point U+338D ㎍ SQUARE MU G for use in CJK contexts. [3] In other contexts, a sequence of the Greek letter mu (U+03BC) and Latin letter g (U+0067) should be used.