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| name = Weight
| width =
| image =
| imagesize =
| caption = A
| unit = [[newton (unit)|newton]] (N)
| otherunits = [[pound-force]] (lbf)
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}}
}}
In [[science]] and [[engineering]], the '''weight''' of an object is a quantity associated with the [[gravitational force]]
|title=Weight and gravity - the need for consistent definitions
|author=Richard C. Morrison
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}}</ref><ref name="Gat">{{cite book |title=Standardization of Technical Terminology: Principles and Practice – ''second volume'' |editor=Richard Alan Strehlow |date=1988 |publisher=[[ASTM International]] |isbn=978-0-8031-1183-7 |chapter=The weight of mass and the mess of weight |last=Gat |first=Uri |pages=45–48 |chapter-url=https://books.google.com/books?id=CoB5w9Km0mUC&pg=PA45}}</ref>
Some standard textbooks<ref name='Knight'>{{Cite book|author=Knight, Randall D.|year=2004|title=Physics for Scientists and Engineers: a Strategic Approach|location=San Francisco,
The [[unit of measurement]] for weight is that of [[force]], which in the [[International System of Units]] (SI) is the [[newton (unit)|newton]].<ref name="Morrison" /> For example, an object with a mass of one kilogram has a weight of about 9.8 newtons on the surface of the Earth, and about one-sixth as much on the [[Moon]]. Although weight and mass are scientifically distinct quantities, the terms are often confused with each other in everyday use (e.g. comparing and converting force weight in pounds to mass in kilograms and vice versa).<ref name="Canada">The National Standard of Canada, CAN/CSA-Z234.1-89 Canadian Metric Practice Guide, January 1989:
*'''5.7.3''' Considerable confusion exists in the use of the term "weight". In commercial and everyday use, the term "weight" nearly always means mass. In science and technology "weight" has primarily meant a force due to gravity. In scientific and technical work, the term "weight" should be replaced by the term "mass" or "force", depending on the application.
*'''5.7.4''' The use of the verb "to weigh" meaning "to determine the mass of", e.g., "I weighed this object and determined its mass to be 5{{spaces}}kg," is correct.</ref>
Further complications in elucidating the various concepts of weight have to do with the [[theory of relativity]] according to which gravity is modeled as a consequence of the [[curvature
==History==
Discussion of the concepts of heaviness (weight) and lightness (levity) date back to the [[ancient Greek philosophy|ancient Greek philosophers]]. These were typically viewed as inherent properties of objects. [[Plato]] described weight as the natural tendency of objects to seek their kin. To [[Aristotle]], weight and levity represented the tendency to restore the natural order of the basic elements: air, earth, fire and water. He ascribed absolute weight to earth and absolute levity to fire. [[Archimedes]] saw weight as a quality opposed to [[buoyancy]], with the conflict between the two determining if an object sinks or floats. The first operational definition of weight was given by [[Euclid]], who defined weight as: "the heaviness or lightness of one thing, compared to another, as measured by a balance."<ref name="Galili"/> Operational balances (rather than definitions) had, however, been around much longer.<ref>http://www.averyweigh-tronix.com/museum {{Webarchive|url=https://web.archive.org/web/20130228235853/http://www.averyweigh-tronix.com/museum/ |date=2013-02-28 }} accessed 29 March 2013.</ref>
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==Definitions==
{{Excessive examples|date=October 2023}}
Several definitions exist for ''weight'', not all of which are equivalent.<ref name="Gat"/><ref name="King">{{cite journal
|title=Weight and weightlessness
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|Resolution 2 of the 3rd General Conference on Weights and Measures<ref name="3rdCGPM"/><ref name="NIST330">{{Cite book |editor1=David B. Newell |editor2=Eite Tiesinga |title=The International System of Units (SI) |url=https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf |publisher=[[National Institute of Standards and Technology|NIST]] |location=Gaithersburg, MD |date=2019|edition=NIST Special publication 330, 2019 |page=46 }}</ref>}}
This resolution defines weight as a vector, since force is a vector quantity. However, some textbooks also take weight to be a scalar by defining:
{{blockquote|
The gravitational acceleration varies from place to place. Sometimes, it is simply taken to have a [[standard gravity|standard value]] of {{nowrap|9.80665 m/s<sup>2</sup>}}, which gives the [[standard weight]].<ref name="3rdCGPM">{{cite web
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The force whose magnitude is equal to ''mg'' newtons is also known as the '''m kilogram weight''' (which term is abbreviated to '''kg-wt''')<ref>Chester, W. Mechanics. George Allen & Unwin. London. 1979. {{ISBN|0-04-510059-4}}. Section 3.2 at page 83.</ref>
{{multiple image▼
| align = right▼
| direction = horizontal▼
| header = Measuring weight versus mass▼
| image1 = Weegschaal-1.jpg▼
| width1 = 125▼
| image2 = Bascula_9.jpg▼
| width2 = 220▼
| footer = Left: A [[Weighing scale|spring scale]] measures weight, by seeing how much the object pushes on a spring (inside the device). On the Moon, an object would give a lower reading. Right: A [[weighing scale|balance scale]] indirectly measures mass,<!-- It compares weights. It has the secondary effect of comparing masses because weight is proportional to mass. --> by comparing an object to references. On the Moon, an object would give the same reading, because the object and references would ''both'' become lighter.}}▼
===Operational definition===
▲{{multiple image
▲| align = right
▲| direction = horizontal
▲| header = Measuring weight versus mass
▲| image1 = Weegschaal-1.jpg
▲| width1 = 125
▲| image2 = Bascula_9.jpg
▲| width2 = 220
▲| footer = Left: A [[Weighing scale|spring scale]] measures weight, by seeing how much the object pushes on a spring (inside the device). On the Moon, an object would give a lower reading. Right: A [[weighing scale|balance scale]] indirectly measures mass,<!-- It compares weights. It has the secondary effect of comparing masses because weight is proportional to mass. --> by comparing an object to references. On the Moon, an object would give the same reading, because the object and references would ''both'' become lighter.
}}
In the operational definition, the weight of an object is the [[force]] measured by the operation of weighing it, which is '''the force it exerts on its support'''.<ref name="King"/> Since ''W'' is the downward force on the body by the centre of earth and there is no acceleration in the body, there exists an opposite and equal force by the support on the body. Also it is equal to the force exerted by the body on its support because action and reaction have same numerical value and opposite direction. This can make a considerable difference, depending on the details; for example, an object in [[free fall]] exerts little if any force on its support, a situation that is commonly referred to as [[weightlessness]]. However, being in free fall does not affect the weight according to the gravitational definition. Therefore, the operational definition is sometimes refined by requiring that the object be at rest.{{Citation needed|date=May 2010}} However, this raises the issue of defining "at rest" (usually being at rest with respect to the Earth is implied by using [[standard gravity]]).{{Citation needed|date=May 2010}} In the operational definition, the weight of an object at rest on the surface of the Earth is lessened by the effect of the centrifugal force from the Earth's rotation.
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==Mass==
{{Main|Mass versus weight}}[[File:WeightNormal.svg|thumb|250px|An object with mass ''m'' resting on a surface and the corresponding [[free body diagram]] of just the object showing the [[force]]s acting on it.
In modern scientific usage, weight and [[mass]] are fundamentally different quantities: mass is an [[Intrinsic and extrinsic properties|intrinsic]] property of [[matter]], whereas weight is a ''force'' that results from the action of [[gravity]] on matter: it measures how strongly the force of gravity pulls on that matter. However, in most practical everyday situations the word "weight" is used when, strictly, "mass" is meant.<ref name="Canada"/><ref name="NIST811wt">{{cite journal |author=A. Thompson |author2=B. N. Taylor |name-list-style=amp |title=The NIST Guide for the use of the International System of Units, Section 8: Comments on Some Quantities and Their Units |journal=Special Publication 811 |url=http://physics.nist.gov/Pubs/SP811/sec08.html#8.3 |publisher=[[NIST]] |orig-year=July 2, 2009 |date=March 3, 2010 |access-date=2010-05-22}}</ref> For example, most people would say that an object "weighs one kilogram", even though the kilogram is a unit of mass.
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| publisher = Chemical Rubber Publishing Co.
| date = 1961
| location = Cleveland,
| pages=3480–3485
}}</ref> at different locations on Earth (see [[Earth's gravity]]). These variations alter the relationship between weight and mass, and must be taken into account in high-precision weight measurements that are intended to indirectly measure mass. [[Spring scale]]s, which measure local weight, must be calibrated at the location at which the objects will be used to show this standard weight, to be legal for commerce.{{Citation needed|date=May 2010|reason=Doesn't this depend on the jurisdiction?}}
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In commercial and everyday use, the term "weight" is usually used to mean mass, and the verb "to weigh" means "to determine the mass of" or "to have a mass of". Used in this sense, the proper SI unit is the [[kilogram]] (kg).<ref name=NIST811wt/>
===Pound and other non-SI units===
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| url = https://www.nist.gov/pml/wmd/metric/common-conversion-b.cfm
| title = Common Conversion Factors, Approximate Conversions from U.S. Customary Measures to Metric
| journal =
| date = 13 January 2010
| publisher = [[National Institute of Standards and Technology]]
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==Relative weights on the Earth and other celestial bodies==
{{Main|Earth's gravity|Surface gravity}}
The table below shows comparative [[Surface gravity|gravitational accelerations at the surface]] of the Sun, the Earth's moon, each of the planets in the solar system. The
{| class="wikitable"
|-
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{{Wiktionary|gross weight}}
* {{annotated link|Human body weight}}
* {{annotated link|Specific weight}}
* [[Tare weight]]
* {{annotated link|wey (unit)|weight}} the English unit
* [[Weight (object)]]
==Notes==
|