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SI is a coherent system. This means that units are always combined without conversion factors. The derived unit of velocity is the meter per second (m/s); the derived unit of volume is the cubic meter (m3). If you know that pressure is force per unit area, then you know that the SI unit of pressure (the pascal) is the unit of force divided by the unit of area and is therefore equal to 1 newton per square meter.
The metric prefixes can be attached to any unit in order to make a unit of a more convenient size. The symbol for the prefix is attached to the symbol for the unit, e.g., nanometer (nm) = 10−9 m. The units of mass are specified in terms of the gram, e.g., microgram (μg) = 10−9 kg.
Only one prefix is used with a unit; the use of units such as the millimicrometer is no longer acceptable. When a unit is raised to a power, the power applies to the prefix as well, e.g., a cubic millimeter (mm3) = 10−9 m3. When a prefix is used with a ratio unit, it should be in the numerator rather than in the denominator, e.g., kilometers/second (km/s) rather than meters/millisecond (m/ms). Only prefixes denoting powers of 103 are normally used. Hecto-, deka-, deci-, and centi- are usually attached only to the metric system units gram, meter, and liter.
Owing to the force of tradition, one noncoherent unit, the liter, equal to 10−3 m3, or 1 dm3, is generally accepted for use with SI. The internationally accepted abbreviation for liter is the letter l; however, this can be confused with the numeral 1, especially in typescript. For this reason, the capital letter L is also used as a symbol for liter. The lower case letter is generally used with prefixes, e.g., dl, ml, fl. The symbols for all other SI units begin with a capital letter if the unit is named after a person and with a lower case letter otherwise. The name of a unit is never capitalized.
kelvin/kel·vin/ (K) (kel´vin) the base SI unit of temperature, equal to 1/273.16 of the absolute temperature of the triple point of water.
See also: Kelvin scale
scaledevice marked at regular intervals, used for measuring;
celsius scale; centigrade scale; °C temperature scale with 100 gradations between freezing (0°C) and boiling points (100°C) of pure water, when recorded at sea level
fahrenheit scale; °F temperature scale with 180 gradations between freezing (32°F) and boiling points (212°F) of pure water, when recorded at sea level
kelvin scale; K temperature scale measured in degrees Celsius from absolute zero; 0K is equivalent to -273.16°C, 0°C equates to 273K, and 100°C equates to 373K