Weight 33931 224723328 2008-07-10T03:29:09Z Senator Palpatine 7357381 Reverted edits by [[Special:Contributions/125.60.243.71|125.60.243.71]] ([[User talk:125.60.243.71|talk]]) to last version by Rracecarr {{otheruses}} [[Image:Weeghaak.JPG|thumb|100px|A [[spring scale]] measures the weight of an object]] In the [[physical science]]s, '''weight''' is a [[measurement]] of the gravitational [[force]] acting on an object.<ref name="Canada" /> Near the surface of the [[Earth]], the [[Earth's gravity|acceleration due to gravity]] is approximately constant; this means that an object's weight is roughly proportional to its [[mass]]. == Weight and mass == {{main|Mass versus weight}} In commerce and in many other applications, weight means the same as [[mass]] as that term is used in physics.<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&nbsp;kg," is correct. </ref><ref name=NIST811wt>Barry N. Taylor, ''Guide for the Use of the International System of Units (SI)'', 1995, [[NIST]] Special Publication 881, section 8.3[http://physics.nist.gov/Pubs/SP811/sec08.html#8.3]</ref> In modern scientific usage, however, weight and mass are fundamentally different quantities: mass is an intrinsic property of [[matter]], whereas weight is a ''force'' that results from the action of [[gravity]] on matter: it measures how strongly gravity pulls on that matter. However, the recognition of this difference is, historically, a relatively recent development and in many everyday situations the word "weight" continues to be used when "mass" is meant. For example, we say that an object "weighs one kilogram", even though the kilogram is a unit of mass. The distinction between mass and weight is unimportant for many practical purposes because the strength of gravity is very simliar everywhere on the surface of the Earth. In such a constant gravitational field, the gravitational force exerted on an object (its weight) is [[Proportionality (mathematics)|directly proportional]] to its mass. So, if object A weighs, say, 10 times as much as object B, then object A's mass is 10 times that of object B. This means that an object's mass can be measured indirectly by its weight (for conversion formulas see [[#Conversion between weight (force) and mass|below]]). For example, when we buy a bag of sugar we can measure its weight (how hard it presses down on the scales) and be sure that this will give a good indication of the quantity that we are actually interested in, which is the mass of sugar in the bag. Nevertheless, the Earth's gravitational field can vary by as much as 0.5%<ref>{{cite book | last = Hodgeman | first = Charles, Ed. | authorlink = | coauthors = | title = Handbook of Chemistry and Physics, 44th Ed. | publisher = Chemical Rubber Publishing Co. | date = 1961 | location = Cleveland, USA | pages = | url = | doi = | id = | isbn = }} p.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. To eliminate this variation, when the weight of objects is used in commerce, the value given is what they would weigh at a nominal [[Standard gravity|standard gravitational acceleration]] of 9.80665 m/s<sup>2</sup> (approx. 32.174 ft/s<sup>2</sup>) [[Spring scale]]s, which measure local weight, must be calibrated at the location at which they will be used to show this standard weight, to be legal for commerce. The use of "weight" for "mass" also persists in some scientific terminology – for example, in the [[chemistry|chemical]] terms "atomic weight", "molecular weight", and "formula weight", rather than the preferred "[[atomic mass]]" etc. The difference between mass and force may be important when: * objects are compared in different gravitational fields, such as away from the Earth's surface. For example, on the surface of the [[Moon]], gravity is only about one-sixth as strong as on the surface of the Earth. A one-kilogram mass is still a one-kilogram mass (as mass is an intrinsic property of the object) but the downward force due to gravity is only one-sixth of what the object would experience on Earth. * locating the [[center of gravity]] of an object (although if the gravitation field is uniform, the center of gravity will coincide with the center of mass). * an object is submersed in a fluid (for instance, a brick weighs less when placed in water, and helium balloon in the atmosphere appears to have negative weight). == Units of weight == Systems of units of weight (force) and mass have a tangled history, partly because the distinction was not properly understood when many of the units first came into use. {{GravEngAbs}} ===SI units=== In most modern scientific work, physical quantities are measured in [[International System of Units|SI]] units. The SI unit of mass (and hence weight in some everyday senses)<ref>Barry N. Taylor, ''Guide for the Use of the International System of Units (SI)'', 1995, [[NIST]] Special Publication 881, section 8.3[http://physics.nist.gov/Pubs/SP811/sec08.html#8.3] "Thus the SI unit of the quantity weight used in this sense is the kilogram (kg) and the verb 'to weigh' means 'to determine the mass of' or '"to have a mass of.'"</ref> is the [[kilogram]]. The SI unit of force (and hence weight in the [[mechanics]] sense) is the [[newton (unit)|newton]] (N) – which can also be expressed in [[SI base unit]]s as kg·m/s² (kilograms times metres per second squared). The gravitational force exerted on an object is proportional to the mass of the object, so it is reasonable to think of the strength of gravity as measured in terms of force per unit mass, that is, newtons per kilogram (N/kg). However, the unit N/kg resolves to m/s²; (metres per second per second), which is the SI unit of acceleration, and in practice gravitational strength is usually quoted as an acceleration. ===The pound and other non-SI units=== In [[United States customary units]], the pound can be either a unit of force or a unit of mass. Related units used in some distinct, separate subsystems of units include the [[poundal]] and the [[slug (mass)|slug]]. The poundal is defined as the force necessary to accelerate a one-pound object at 1&nbsp;ft/s², and is equivalent to about 1/32 of a pound (force). The slug is defined as the amount of mass that accelerates at 1&nbsp;ft/s² when a pound of force is exerted on it, and is equivalent to about 32 pounds (mass). The [[kilogram-force]] is a non-SI unit of force, defined as the force exerted by a one-kilogram mass in standard Earth gravity (equal to 9.80665 newtons exactly). The [[dyne]] is the [[centimeter-gram-second|cgs]] unit of force and is not a part of SI, while weights measured in the cgs unit of mass, the gram, remain a part of SI. ===Conversion between weight (force) and mass=== To convert between weight (force) and mass we use Newton's second law, ''F'' = ''ma'' (force = mass × acceleration). Here, ''F'' is the force (weight) due to gravity, ''m'' is the mass of the object in question, and ''a'' is the acceleration due to gravity, on Earth approximately 9.8&nbsp;m/s² or 32.2&nbsp;ft/s². In this context the same equation is often written as ''W'' = ''mg'', with ''W'' standing for weight, and ''g'' for the acceleration due to gravity. == Sensation of weight == {{see also|apparent weight}} The weight force that we actually sense is not the downward force of gravity, but the [[normal force]] (an upward [[contact force]]) exerted by the surface we stand on, which opposes gravity and prevents us falling to the center of the Earth. This normal force, called the apparent weight, is the one that is measured by a spring scale. For a body supported in a stationary position, the normal force balances the earth's gravitational force, and so apparent weight has the same magnitude as actual weight. (Technically, things are slightly more complicated. For example, an object immersed in water weighs less, according to a spring scale, than the same object in air; this is due to [[buoyancy]], which opposes the weight force and therefore generates a smaller normal. These and other factors are explained further under apparent weight.) If there is no contact with any surface to provide such an opposing force then there is no sensation of weight (no apparent weight). This happens in [[free-fall]], as experienced by sky-divers (until they approach [[terminal velocity]]) and astronauts in orbit, who feel "[[weightless]]" even though their bodies are still subject to the force of gravity: they're just no longer resisting it. The experience of having no apparent weight is also known as [[microgravity]]. A degree of reduction of apparent weight occurs, for example, in elevators. In an elevator, a spring scale will register a decrease in a person's (apparent) weight as the elevator starts to accelerate downwards. This is because the opposing force of the elevator's floor decreases as it accelerates away underneath one's feet. == Measuring weight == :''Main article: [[Weighing scale]]'' Weight is commonly measured using one of two methods. A [[Weighing scale#Spring scales|spring scale]] or [[Weighing scale#Hydraulic or pneumatic scales|hydraulic or pneumatic scale]] measures local weight, the local [[force]] of [[gravity]] on the object (strictly [[apparent weight|''apparent'' weight force]]). Since the local force of gravity can vary by up to 0.5% at different locations, spring scales will measure slightly different weights for the same object (the same mass) at different locations. To standardize weights, scales are always calibrated to read the weight an object would have at a nominal [[standard gravity]] of 9.80665 m/s<sup>2</sup> (approx. 32.174 ft/s<sup>2</sup>). However, this calibration is done at the factory. When the scale is moved to another location on Earth, the force of gravity will be different, causing a slight error. So to be highly accurate, and legal for commerce, [[spring scale]]s must be recalibrated at the location at which they will be used. A ''[[Weighing scale#Balances|balance]]'' on the other hand, compares the weight of an object to standard weights, using a [[lever]]. Since any variations in gravity will act equally on the unknown and the known weights, a balance will indicate the same value at any location on Earth. Therefore balance weights are usually calibrated in [[mass]] units, so the balance measures mass. Balances can be marked in weight units, but since the weights are calibrated at the factory for [[standard gravity]], the balance will measure standard weight, what the object would weigh at [[standard gravity]], not the actual local force of gravity on the object. If the actual force of gravity on the object is needed, this can be calculated by multiplying the mass measured by the balance by the acceleration due to gravity – either [[standard gravity]] (for everyday work) or the precise local gravity (for precision work). Tables of the gravitational acceleration at different locations can be found on the web. '''Gross weight''' is a term that generally is found in commerce or trade applications, and refers to the total weight of a product and its packaging. Conversely, '''net weight''' refers to the weight of the product alone, discounting the weight of its container or packaging. ==Relative weights on the Earth, other planets and the Moon== {{main|Earth's gravity}} The table below shows comparative gravitational accelerations at the surface of the Sun, the Earth's moon, each of the planets in the solar system, and Pluto. The “surface” is taken to mean the cloud tops of the [[gas giants]] (Jupiter, Saturn, Uranus and Neptune). For the Sun, the surface is taken to mean the [[photosphere]]. The values in the table have not been de-rated for the centrifugal effect of planet rotation (and cloud-top wind speeds for the gas giants) and therefore, generally speaking, are similar to the actual gravity that would be experienced near the poles.<br><br> {| border="1" cellspacing="0" cellpadding="4" |- ! Body ! Multiple of<br>Earth gravity ! m/s² |- | [[Sun]] | 27.90 | 274.1 |- | [[Mercury (planet)|Mercury]] | 0.3770 | 3.703 |- | [[Venus (planet)|Venus]] | 0.9032 | 8.872 |- | [[Earth (planet)|Earth]] | 1 (by definition) | 9.8226<span style="margin-left:0.2em"><ref></span>This value excludes the adjustment for centrifugal force due to Earth’s rotation and is therefore greater than the 9.806<span style="margin-left:0.25em">65&nbsp;m/s²<span> value of [[standard gravity]].</ref> |- | [[Moon]] | 0.1655 | 1.625 |- | [[Mars]] | 0.3895 | 3.728 |- | [[Jupiter]] | 2.640 | 25.93 |- | [[Saturn]] | 1.139 | 11.19 |- | [[Uranus]] | 0.917 | 9.01 |- | [[Neptune]] | 1.148 | 11.28 |- | [[Pluto]] | 0.0621 | 0.610 |} ==References== <!-- ---------------------------------------------------------- See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for a discussion of different citation methods and how to generate footnotes using the <ref>, </ref> and <reference /> tags http://quest.arc.nasa.gov/saturn/qa/saturn/ ----------------------------------------------------------- --> {{reflist}} ==See also== * [[Weights and measures]] * [[Ancient weights and measures]] * [[Medieval weights and measures]] * [[Atomic weight]] * [[Human weight]] * [[Body Mass Index]] * [[Curb weight]] [[Category:Commerce]] [[Category:Mass]] [[Category:Force]] [[Category:Physiology]] [[af:Gewig]] [[ar:وزن]] [[ast:Pesu]] [[bn:ওজন (ভার)]] [[zh-min-nan:Tāng-liōng]] [[be:Вага]] [[bs:Težina]] [[bg:Тегло]] [[ca:Pes]] [[cs:Váha]] [[da:Vægt (fysik)]] [[de:Gewicht]] [[et:Kaal]] [[es:Peso]] [[eo:Pezo]] [[eu:Pisu]] [[fa:وزن]] [[fr:Poids]] [[gl:Peso]] [[hi:भार]] [[ko:무게]] [[hr:Težina]] [[io:Pezo]] [[id:Berat]] [[is:Þyngd]] [[it:Forza peso]] [[he:משקל (פיזיקה)]] [[lv:Svars]] [[lt:Svoris]] [[hu:Súly]] [[mk:Тежина]] [[ml:ഭാരം]] [[ms:Berat]] [[nl:Gewicht]] [[ja:重さ]] [[no:Tyngde]] [[pl:Ciężar]] [[pt:Peso]] [[ksh:Jeweech]] [[ro:Greutate]] [[qu:Llasaq kay]] [[ru:Вес]] [[scn:Pisu]] [[simple:Weight]] [[sk:Gravitačná hmotnosť]] [[sl:Teža]] [[sr:Тежина]] [[fi:Paino]] [[sv:Tyngd]] [[th:น้ำหนัก]] [[vi:Lực hấp dẫn#Tr.E1.BB.8Dng_l.E1.BB.B1c]] [[tr:Ağırlık]] [[uk:Вага]] [[yi:וואג]] [[zh:重量]]