A UNIT OF MEASUREMENT is a definite magnitude of a quantity , defined and adopted by convention or by law, that is used as a standard for measurement of the same quantity. Any other value of that quantity can be expressed as a simple multiple of the unit of measurement. For example, length is a physical quantity . The metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre". The definition, agreement, and practical use of units of measurement
have played a crucial role in human endeavour from early ages up to
this day. Different systems of units used to be very common. Now there
is a global standard, the
International System of Units
In trade, WEIGHTS AND MEASURES is often a subject of governmental
regulation, to ensure fairness and transparency. The International
Bureau of Weights and Measures (BIPM) is tasked with ensuring
worldwide uniformity of measurements and their traceability to the
International System of Units
Metrology is the science for developing nationally and internationally accepted units of weights and measures. In physics and metrology, units are standards for measurement of physical quantities that need clear definitions to be useful. Reproducibility of experimental results is central to the scientific method . A standard system of units facilitates this. Scientific systems of units are a refinement of the concept of weights and measures developed long ago for commercial purposes.
Science
In the social sciences , there are no standard units of measurement and the theory and practice of measurement is studied in psychometrics and the theory of conjoint measurement . CONTENTS * 1 History * 2 Systems of units * 2.1 Traditional systems * 2.2 Metric systems * 2.3 Natural systems * 2.4 Legal control of weights and measures * 3 Base and derived units * 4 Calculations with units of measurement * 4.1 Units as dimensions * 4.2 Expressing a physical value in terms of another unit * 5 Realworld implications * 6 See also * 7 Notes * 8 External links HISTORY Main article:
History of measurement
A unit of measurement is a standardised quantity of a physical
property, used as a factor to express occurring quantities of that
property.
Units of measurement
The earliest known uniform systems of weights and measures seem to
have all been created sometime in the 4th and 3rd millennia BC among
the ancient peoples of
Mesopotamia
Weights and measures are mentioned in the
Bible
In the
Magna Carta
As of the 21st Century, multiple unit systems are used all over the
world such as the
United States
SYSTEMS OF UNITS Main article: System of measurement TRADITIONAL SYSTEMS Historically many of the systems of measurement which had been in use were to some extent based on the dimensions of the human body. As a result, units of measure could vary not only from location to location, but from person to person. METRIC SYSTEMS A number of metric systems of units have evolved since the adoption
of the original metric system in
France
Both the imperial units and
US customary units
NATURAL SYSTEMS While the above systems of units are based on arbitrary unit values, formalised as standards, some unit values occur naturally in science. Systems of units based on these are called natural units . Similar to natural units, atomic units (au) are a convenient system of units of measurement used in atomic physics . Also a great number of unusual and nonstandard units may be encountered. These may include the solar mass (7030200000000000000♠2×1030 kg) and the megaton (the energy released by detonating one million tons of trinitrotoluene , TNT). LEGAL CONTROL OF WEIGHTS AND MEASURES Main articles: Weights and Measures Act and Trading standards To reduce the incidence of retail fraud, many national statutes have standard definitions of weights and measures that may be used (hence "statute measure"), and these are verified by legal officers. BASE AND DERIVED UNITS Different systems of units are based on different choices of a set of base units . The most widely used system of units is the International System of Units, or SI . There are seven SI base units . All other SI units can be derived from these base units. For most quantities a unit is absolutely necessary to communicate values of that physical quantity. For example, conveying to someone a particular length without using some sort of unit is impossible, because a length cannot be described without a reference used to make sense of the value given. But not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as combinations of units of other quantities. Thus only a small set of units is required. These units are taken as the base units. Other units are derived units. Derived units are a matter of convenience, as they can be expressed in terms of basic units. Which units are considered base units is a matter of choice. The base units of SI are actually not the smallest set possible. Smaller sets have been defined. For example, there are unit sets in which the electric and magnetic field have the same unit. This is based on physical laws that show that electric and magnetic field are actually different manifestations of the same phenomenon. CALCULATIONS WITH UNITS OF MEASUREMENT UNITS AS DIMENSIONS Any value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Z is expressed as the product of a unit and a numerical factor: Z = n = n . {displaystyle Z=ntimes =n.} For example, "2 candlesticks" Z = 2 . The multiplication sign is usually left out, just as it is left out between variables in scientific notation of formulas. The conventions used to express quantities is referred to as quantity calculus . In formulas the unit can be treated as if it were a specific magnitude of a kind of physical dimension : see dimensional analysis for more on this treatment. Units can only be added or subtracted if they are the same type; however units can always be multiplied or divided, as George Gamow used to explain: "2 candlesticks" times "3 cabdrivers" = 6 . A distinction should be made between units and standards. A unit is
fixed by its definition, and is independent of physical conditions
such as temperature. By contrast, a standard is a physical realization
of a unit, and realizes that unit only under certain physical
conditions. For example, the metre is a unit, while a metal bar is a
standard.
One
There are certain rules that have to be used when dealing with units:
* Treat units algebraically. Only add like terms. When a unit is
divided by itself, the division yields a unitless one. When two
different units are multiplied or divided, the result is a new unit,
referred to by the combination of the units. For instance, in SI, the
unit of speed is metres per second (m/s). See dimensional analysis . A
unit can be multiplied by itself, creating a unit with an exponent
(e.g. m2/s2). Put simply, units obey the laws of indices. (See
Exponentiation
EXPRESSING A PHYSICAL VALUE IN TERMS OF ANOTHER UNIT
Conversion of units
Starting with: Z = n i i {displaystyle Z=n_{i}times _{i}} just replace the original unit i {displaystyle _{i}} with its meaning in terms of the desired unit j {displaystyle _{j}} , e.g. if i = c i j j {displaystyle _{i}=c_{ij}times _{j}} , then: Z = n i ( c i j j ) = ( n i c i j ) j {displaystyle Z=n_{i}times (c_{ij}times _{j})=(n_{i}times c_{ij})times _{j}} Now n i {displaystyle n_{i}} and c i j {displaystyle c_{ij}} are both numerical values, so just calculate their product. Or, which is just mathematically the same thing, multiply Z by unity, the product is still Z: Z = n i i ( c i j j / i ) {displaystyle Z=n_{i}times _{i}times (c_{ij}times _{j}/_{i})} For example, you have an expression for a physical value Z involving the unit feet per second ( i {displaystyle _{i}} ) and you want it in terms of the unit miles per hour ( j {displaystyle _{j}} ): * Find facts relating the original unit to the desired unit: 1 mile = 5280 feet and 1 hour = 3600 seconds * Next use the above equations to construct a fraction that has a value of unity and that contains units such that, when it is multiplied with the original physical value, will cancel the original units: 1 = 1 m i 5280 f t a n d 1 = 3600 s 1 h {displaystyle 1={frac {1,mathrm {mi} }{5280,mathrm {ft} }}quad mathrm {and} quad 1={frac {3600,mathrm {s} }{1,mathrm {h} }}} * Last, multiply the original expression of the physical value by the fraction, called a conversion factor , to obtain the same physical value expressed in terms of a different unit. Note: since valid conversion factors are dimensionless and have a numerical value of one , multiplying any physical quantity by such a conversion factor (which is 1) does not change that physical quantity. 52.8 f t s = 52.8 f t s 1 m i 5280 f t 3600 s 1 h = 52.8 3600 5280 m i / h = 36 m i / h {displaystyle 52.8,{frac {mathrm {ft} }{mathrm {s} }}=52.8,{frac {mathrm {ft} }{mathrm {s} }}{frac {1,mathrm {mi} }{5280,mathrm {ft} }}{frac {3600,mathrm {s} }{1,mathrm {h} }}={frac {52.8times 3600}{5280}},mathrm {mi/h} =36,mathrm {mi/h} } Or as an example using the metric system, you have a value of fuel economy in the unit litres per 100 kilometres and you want it in terms of the unit microlitres per metre: 9 L 100 k m = 9 L 100 k m 1000000 L 1 L 1 k m 1000 m = 9 1000000 100 1000 L / m = 90 L / m {displaystyle mathrm {frac {9,{rm {L}}}{100,{rm {km}}}} =mathrm {frac {9,{rm {L}}}{100,{rm {km}}}} mathrm {frac {1000000,{rm {mu L}}}{1,{rm {L}}}} mathrm {frac {1,{rm {km}}}{1000,{rm {m}}}} ={frac {9times 1000000}{100times 1000}},mathrm {mu L/m} =90,mathrm {mu L/m} } REALWORLD IMPLICATIONS
One
On 15 April 1999,
Korean Air cargo flight 6316 from
Shanghai
In 1983, a Boeing 767 (which came to be known as the
Gimli Glider )
ran out of fuel in midflight because of two mistakes in figuring the
fuel supply of
Air Canada
SEE ALSO *
List of humorous units of measurement
*
List of unusual units of measurement
*
GNU Units
*
Unified Code for Units of Measure
*
United States
* v * t * e Systems of measurement CURRENT GENERAL *
International System of Units
SPECIFIC * Apothecaries\' * Avoirdupois * Troy * Astronomical * Electrical * Temperature NATURAL * atomic * geometrised * Gaussian * Lorentz–Heaviside * Planck * quantum chromodynamical * Stoney BACKGROUND METRIC * Overview * Introduction * Outline * History * Metrication UK/US * Overview * Comparison * Foot–pound–second (FPS) HISTORIC METRIC * metre–kilogram–second (MKS) * metre–tonne–second (MTS) * centimetre–gram–second (CGS) * gravitational * quadrant–eleventhgram–second (QES) (hebdometre–undecimogramme–second (HUS)) EUROPE * Byzantine
* Cornish
* Cypriot
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* Danish
* Dutch
* English
* Estonian
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* French (Trad. •
Mesures usuelles
ASIA * Afghan * Cambodian * Chinese * Hindu * Hong Kong * India * Indonesian * Japanese * Korean * Mongolian * Omani * Philippine * Pegu * Singaporean * Sri Lankan * Syrian * Taiwanese * Tatar * Thai * Vietnamese AFRICA * Algerian * Ethiopian * Egyptian * Eritrean * Guinean * Libyan * Malagasy * Mauritian * Moroccan * Seychellois * Somalian * South African * Tunisian * Tanzanian NORTH AMERICA * Costa Rican * Cuban * Haitian * Honduran * Mexico * Nicaraguan * Puerto Rican SOUTH AMERICA * Argentine * Brazilian * Chilean * Colombian * Paraguayan * Peruvian * Uruguayan * Venezuelan ANCIENT * Arabic * Biblical and Talmudic * Egyptian * Greek * Hindu * Indian * Mesopotamian * Persian * Roman LIST ARTICLES * Humorous ( FFF system ) * Obsolete * Unusual OTHER * Nbody * Modulor NOTES * ^ "measurement unit", in International Vocabulary of Metrology
– Basic and General Concepts and Associated Terms (VIM) (PDF) (3rd
ed.), Joint Committee for Guides in Metrology, 2008, pp. 6–7 .
* ^ Yunus A. Çengel & Michael A. Boles (2002). Thermodynamics: An
Engineering
