Length: Difference between revisions

| symbols = ''{{mvar|l''}}

| dimension = '''<math>\mathsf{L'''}</math>

'''Length''' is a measure of [[distance]]. In the [[International System of Quantities]], length is a [[quantity]] with [[Dimension (physical quantity)|dimension]] distance. In most [[Measurement system|systems of measurement]] a [[Base unit (measurement)|base unit]] for length is chosen, from which all other units are derived. In the [[International System of Units]] (SI) system the base unit for length is the [[metre]].<ref name=":0" />

Length is commonly understood to mean the most extended [[size|dimension]] of a fixed object.<ref name=":0">{{Cite webencyclopedia|url=http://wordnetweb.princeton.edu/perl/webwn?s=length|titleencyclopedia=WordNet Search - 3.1|websitetitle=wordnetweb.princeton.eduLength|access-date=15 March 2020|archive-url=https://web.archive.org/web/20160925172753/http://wordnetweb.princeton.edu/perl/webwn?s=LENGTH|archive-date=25 September 2016|url-status=live}}</ref> However, this is not always the case and may depend on the position the object is in.

Various terms for the length of a fixed object are used, and these include [[height]], which is vertical length or vertical extent, and '''width''', '''breadth''', orand depth. ''Height'depth'''. Height is used when there is a base from which vertical measurements can be taken. ''Width'' orand ''breadth'' usually refer to a shorter dimension whenthan ''length is the longest one''. ''Depth'' is used for the third dimensionmeasure of a three[[third dimensional objectdimension]].<ref>{{Cite web|url=http://thinkmath.edc.org/resource/measurement-length-width-height-depth|title=Measurement: Length, width, height, depth &#124; |website=Think Math!|website=thinkmath.edc.org|access-date=15 March 2020|archive-url=https://web.archive.org/web/20200224001255/http://thinkmath.edc.org/resource/measurement-length-width-height-depth|archive-date=24 February 2020|url-status=live}}</ref>

== History ==

The [[volume]] of a [[SolidRectangular cuboid|solid rectangular box]] (such as a [[plank of wood]]) is often described as length × height × depth.

In the physical sciences and engineering, when one speaks of {{em|[[unit of length|units of length]]}}, the word {{em|length}} is synonymous with [[distance]]. There are several [[Units of measurement|units]] that are used to [[Measurement|measure]] length. Historically, units of length may have been derived from the lengths of human body parts, the distance traveledtravelled in a number of paces, the distance between landmarks or places on the Earth, or arbitrarily on the length of some common object.

In the [[International System of Units]] (SI), the [[SI base unit|base unit]] of length is the [[metre]] (symbol, m) and is, now defined in terms of the [[speed of light]] (about 300 million metres per [[second]]). The [[millimetre]] (mm), [[centimetre]] (cm) and the [[kilometre]] (km), derived from the metre, are also commonly used units. In [[U.S. customary units]], English or [[Imperialimperial system of units]], commonly used units of length are the [[inch]] (in), the [[foot (length)|foot]] (ft), the [[yard]] (yd), and the [[statute mile|mile]] (mi). A unit of length used in [[navigation]] is the [[nautical mile]] (nmi).<ref>{{cite book|last=Cardarelli|first=François|title=Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins|url=https://archive.org/details/encyclopaediaofs0000card|url-access=registration|year=2003|publisher=Springer|isbn=9781852336820 }}</ref>

Units used to denote distances in the vastness of space, as in [[astronomy]], are much longer than those typically used on Earth (metre or centimetrekilometre) and include the [[astronomical unit]] (au), the [[light-year]], and the [[parsec]] (pc).

Units used to denote sub-atomic distances, as in [[nuclear physics]], are much smaller than the centimetremillimetre. Examples include the [[fermi (unit)|fermi]] (fm).