Editing Mass (section)

=== Weight vs. mass ===
{{main|Mass versus weight}}
[[File:Mass versus weight in earth and mars.svg|300px|right|thumb|Mass and weight of a given object on [[Earth]] and [[Mars]]. Weight varies due to different amount of [[gravitational acceleration]] whereas mass stays the same.]]
In everyday usage, mass and "[[weight]]" are often used interchangeably. For instance, a person's weight may be stated as 75&nbsp;kg. In a constant gravitational field, the weight of an object is proportional to its mass, and it is unproblematic to use the same unit for both concepts. But because of slight differences in the strength of the [[Gravity of Earth|Earth's gravitational field]] at different places, the [[Mass versus weight|distinction]] becomes important for measurements with a precision better than a few percent, and for places far from the surface of the Earth, such as in space or on other planets. Conceptually, "mass" (measured in [[kilograms]]) refers to an intrinsic property of an object, whereas "weight" (measured in [[newtons]]) measures an object's resistance to deviating from its current course of [[free fall]], which can be influenced by the nearby gravitational field. No matter how strong the gravitational field, objects in free fall are [[Weightlessness|weightless]], though they still have mass.<ref>{{cite news |last=Kane |first=Gordon |title=The Mysteries of Mass |newspaper=Scientific American |pages=32–39 |publisher=Nature America, Inc. |date=4 September 2008 |url=http://www.scientificamerican.com/article.cfm?id=the-mysteries-of-mass |access-date=2013-07-05}}</ref>

The force known as "weight" is proportional to mass and [[acceleration]] in all situations where the mass is accelerated away from free fall. For example, when a body is at rest in a gravitational field (rather than in free fall), it must be accelerated by a force from a scale or the surface of a planetary body such as the [[Earth]] or the [[Moon]]. This force keeps the object from going into free fall. Weight is the opposing force in such circumstances and is thus determined by the acceleration of free fall. On the surface of the Earth, for example, an object with a mass of 50&nbsp;kilograms weighs 491 newtons, which means that 491 newtons is being applied to keep the object from going into free fall. By contrast, on the surface of the Moon, the same object still has a mass of 50&nbsp;kilograms but weighs only 81.5&nbsp;newtons, because only 81.5 newtons is required to keep this object from going into a free fall on the moon. Restated in mathematical terms, on the surface of the Earth, the weight ''W'' of an object is related to its mass ''m'' by {{nowrap|1=''W'' = ''mg''}}, where {{nowrap|1=''g'' = {{val|fmt=commas|9.80665|u=m/s<sup>2</sup>}}}} is the acceleration due to [[Earth's gravity|Earth's gravitational field]], (expressed as the acceleration experienced by a free-falling object).

For other situations, such as when objects are subjected to mechanical accelerations from forces other than the resistance of a planetary surface, the weight force is proportional to the mass of an object multiplied by the total acceleration away from free fall, which is called the [[proper acceleration]]. Through such mechanisms, objects in elevators, vehicles, centrifuges, and the like, may experience weight forces many times those caused by resistance to the effects of gravity on objects, resulting from planetary surfaces. In such cases, the generalized equation for weight ''W'' of an object is related to its mass ''m'' by the equation {{nowrap|1=''W'' = –''ma''}}, where ''a'' is the proper acceleration of the object caused by all influences other than gravity. (Again, if gravity is the only influence, such as occurs when an object falls freely, its weight will be zero).