Editing Weight (section)

==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. The magnitude of force that the table is pushing upward on the object (the ''N'' vector) is equal to the downward force of the object's weight (shown here as ''mg'', as weight is equal to the object's mass multiplied with the acceleration due to gravity): because these forces are equal, the object is in a state of [[mechanical equilibrium|equilibrium]] (all the forces and [[Moment (physics)|moments]] acting on it sum to zero).]]
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.

The distinction between mass and weight is unimportant for many practical purposes because the strength of gravity does not vary too much on the surface of the Earth. In a uniform gravitational field, the gravitational force exerted on an object (its weight) is [[Proportionality (mathematics)|directly proportional]] to its mass. For example, object A weighs 10 times as much as object B, so therefore the mass of object A is 10 times greater than that of object B. This means that an object's mass can be measured indirectly by its weight, and so, for everyday purposes, [[weighing]] (using a [[weighing scale]]) is an entirely acceptable way of measuring mass. Similarly, a [[Weighing scale#Balance|balance]] measures mass indirectly by comparing the weight of the measured item to that of an object(s) of known mass. Since the measured item and the comparison mass are in virtually the same location, so experiencing the same [[gravity|gravitational field]], the effect of varying gravity does not affect the comparison or the resulting measurement.

The Earth's [[gravity|gravitational field]] is not uniform but can vary by as much as 0.5%<ref>{{cite book
  | editor-last = Hodgeman
  | editor-first = Charles
  | title = Handbook of Chemistry and Physics
  | edition = 44th
  | publisher = Chemical Rubber Publishing Co.
  | date = 1961
  | location = Cleveland, US
  | 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?}}

This table shows the variation of acceleration due to gravity (and hence the variation of weight) at various locations on the Earth's surface.<ref>{{cite book
|first = John B
|last = Clark
|title = Physical and Mathematical Tables
|publisher = Oliver and Boyd
|date = 1964}}</ref>

{| class="wikitable"
|-
! Location
! Latitude
! m/s<sup>2</sup>
!Absolute difference from equator
!Percentage difference from equator
|-
| [[Equator]]
| 0°
| 9.7803
|0.0000
|0%
|-
| [[Sydney]]
| 33°52′ S
| 9.7968
|0.0165
|0.17%
|-
| [[Aberdeen]]
| 57°9′ N
| 9.8168
|0.0365
|0.37%
|-
| [[North Pole]]
| 90° N
| 9.8322
|0.0519
|0.53%
|}

The historical use of "weight" for "mass" also persists in some scientific terminology – for example, the [[chemistry|chemical]] terms "atomic weight", "molecular weight", and "formula weight", can still be found rather than the preferred "[[atomic mass]]", etc.

In a different gravitational field, for example, on the surface of the [[Moon]], an object can have a significantly different weight than on Earth. The gravity on the surface of the Moon 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, and therefore its weight, is only one-sixth of what the object would have on Earth. So a man of mass 180 [[Pound (mass)|pounds]] weighs only about 30 [[pound-force|pounds-force]] when visiting the Moon.

===SI units===
In most modern scientific work, physical quantities are measured in [[International System of Units|SI]] units. The SI unit of weight is the same as that of force: the [[newton (unit)|newton]] (N) – a derived unit which can also be expressed in [[SI base unit]]s as kg⋅m/s<sup>2</sup> (kilograms times metres per second squared).<ref name=NIST811wt/>

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===
In [[United States customary units]], the pound can be either a unit of force or a unit of mass.<ref>{{cite journal
| 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 = NIST
| date = 13 January 2010
| publisher = [[National Institute of Standards and Technology]]
| access-date = 2013-09-03}}</ref> Related units used in some distinct, separate subsystems of units include the [[poundal]] and the [[slug (unit)|slug]]. The poundal is defined as the force necessary to accelerate an object of one-pound ''mass'' at 1{{spaces}}ft/s<sup>2</sup>, and is equivalent to about 1/32.2 of a pound-''force''. The slug is defined as the amount of mass that accelerates at 1{{spaces}}ft/s<sup>2</sup> when one pound-force is exerted on it, and is equivalent to about 32.2 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 [[centimetre-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.