## Definitions::Weight

### ::concepts

Weight::object    Gravity::force    Scale::earth    Standard::which    Title::measured    Journal::surface

Definitions convert}} in 0.86 seconds. This is a horizontal acceleration of 5.3 g. Combined with the vertical g-force in the stationary case the Pythagorean theorem yields a g-force of 5.4 g. It is this g-force that causes the driver's weight if one uses the operational definition. If one uses the gravitational definition, the driver's weight is unchanged by the motion of the car.

Several definitions exist for weight, not all of which are equivalent.<ref name="Gat"/><ref name="King">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref name="French">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref name="Galili-Lehavi">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

### Gravitational definition

The most common definition of weight found in introductory physics textbooks defines weight as the force exerted on a body by gravity.<ref name="Morrison"/><ref name="Galili-Lehavi"/> This is often expressed in the formula W = mg, where W is the weight, m the mass of the object, and g gravitational acceleration.

In 1901, the 3rd General Conference on Weights and Measures (CGPM) established this as their official definition of weight:

"The word weight denotes a quantity of the same natureUnknown extension tag "ref" as a force: the weight of a body is the product of its mass and the acceleration due to gravity."

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:

Definitions

The gravitational acceleration varies from place to place. Sometimes, it is simply taken to have a standard value of 9.80665 m/s2, which gives the standard weight.<ref name="3rdCGPM">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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>

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### Operational definition

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"/> 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.{{ safesubst:#invoke:Unsubst||date=__DATE__ |\$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }} However, this raises the issue of defining "at rest" (usually being at rest with respect to the Earth is implied by using standard gravity{{ safesubst:#invoke:Unsubst||date=__DATE__ |\$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}). 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.

### ISO definition

In the ISO International standard ISO 80000-4(2006),<ref>ISO 80000-4:2006, Quantities and units - Part 4: Mechanics</ref> describing the basic physical quantities and units in mechanics as a part of the International standard ISO/IEC 80000, the definition of weight is given as:

ISO 80000-4 (2006)

The definition is dependent on the chosen frame of reference. When the chosen frame is co-moving with the object in question then this definition precisely agrees with the operational definition.<ref name="French"/> If the specified frame is the surface of the Earth, the weight according to the ISO and gravitational definitions differ only by the centrifugal effects due to the rotation of the Earth.

### Apparent weight

{{#invoke:main|main}} In many real world situations the act of weighing may produce a result that differs from the ideal value provided by the definition used. This is usually referred to as the apparent weight of the object. A common example of this is the effect of buoyancy, when an object is immersed in a fluid the displacement of the fluid will cause an upward force on the object, making it appear lighter when weighed on a scale.<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> The apparent weight may be similarly affected by levitation and mechanical suspension. When the gravitational definition of weight is used, the operational weight measured by an accelerating scale is often also referred to as the apparent weight.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Weight sections
Intro  History  Definitions  Weight and mass  Sensation of weight  Measuring weight  Relative weights on the Earth and other celestial bodies  See also  Notes  References

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