Editing Weight (section)

===Operational definition===
{{multiple image
| align             = right
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| 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.
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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. 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]].

The operational definition, as usually given, does not explicitly exclude the effects of [[buoyancy]], which reduces the measured weight of an object when it is immersed in a fluid such as air or water. As a result, a floating [[balloon]] or an object floating in water might be said to have zero weight.