'''Weight''' is the force exerted upon an object by virtue of its position in a gravitational field. It is equal to the
mass of the object multiplied by the magnitude of the gravitational field. The word
weight entered Old English sometime aroud the
9th century, and meant the quantity measured with a
balance.
Weight and mass
"Weight" is often used as a synonym for
mass. For instance, when we buy or sell goods "by weight", we are interested in the amount of goods exchanged, not how hard it presses down on the table. Similarly, in measurements of
body weight we are primarily interested in the amount of tissue (fat, muscle, etc.) present. Correspondingly, weight is often given in
kilograms and other units of mass.
In the
physical sciences, people usually distinguish between weight and mass. Under most circumstances, this ambiguity is not a problem, because the weight of an object is
directly proportional to its mass, and the constant of proportionality -- the strength of the gravitational field -- is approximately constant everywhere on the surface of the
Earth (around 9.8 m/s²). For instance, a body will exert less force if it is located on the
Moon than if it is on the Earth, since the gravitational field of the Moon is weaker; its mass, on the other hand, does not depend on position. Although terms such as "atomic weight", "molecular weight", and "formula weight" may still be encountered, such usage is often discouraged; terms like
atomic mass are used instead.
Related to the historical identification of mass and weight, the
pound has been used both as a unit of mass and as a unit of force. In the
United States,
United Kingdom, and elsewhere, the pound is and always has been officially defined as a unit of mass. The corresponding force is called a
pound-force, and similarly the weight of a kilogram of material on Earth is called a
kilogram-force. However, the use of pounds to measure forces is still common in engineering, and it occurs in derived units like
p.s.i. (
pounds per square
inch). In most countries, scientists have adopted
SI units, which use
kilogram for mass and
newton for force non-interchangeably.
Weight as a force
The
SI unit for weight is the
newton (N), or
kilogram metres per second squared (kg m s
−2).
The weight force that we sense is actually the normal
force exerted by the surface we stand on, which prevents us from being pulled to the centre of the Earth, and not the weight itself. This normal force, that we can call the
apparent weight is the one that is measured by a weighing scale,
not the weight itself. A good evidence of this is given by the fact that a person moving up and down on his toes does see the indicator moving, telling that the measured force is changing while his weight, that depends only on his mass, the Earth mass and the distance between his
center of mass and the center of Earth obviously do no change.
In contrast, in free-fall, there is no
apparent weight because we are not in contact with any surface to provide such a normal force. The experience of having no apparent weight is known as
weightlessness or microgravity.
Comparative weights on bodies of the solar system
The following is a list of the weights of a
mass on some of the bodies in the solar system, relative to its weight on Earth:
For weight variations on Earth, see
gee,
physical geodesy and
gravity anomaly.
Human weight in the medical sciences and ordinary language
Although it is technically more accurate to prefer body mass to
body weight, the term
weight is overwhelmingly used in daily English speech and in biological and medical science contexts.
Body weight is measured in
kilograms throughout the world. Most hospitals in the
United States use kilograms for calculations, but use kilograms and
pounds simultaneously for other purposes (a pound is 0.46 kg). Many people in the U.K. still measure their weight using the
stone equal to 14 lb (6.35 kg).
Sports usage
Participants in sports such as
boxing,
wrestling,
judo, and weight-lifting are classified according to their body weight, measured in units of mass such as pounds or kilograms. See, e.g.,
wrestling weight classes,
boxing weight classes,
judo at the 2004 Summer Olympics,
boxing at the 2004 Summer Olympics. In
horse racing, weight is used to
handicap horses.
A
weight also refers to the physical objects used in weight-lifting and other sports such as the
hammer throw.
Related articles
----
For the 1994 album by the group
Rollins Band, see
Weight (album).
Category:Commerce
Category:Mass
Category:Force
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es:Peso
eo:Pezo
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id:Berat
he:משקל
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ja:重さ
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pt:Peso (física)
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zh:重量