In the physical sciences, weight is the interaction of matter with 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 around the 9th century, and meant the quantity measured with a balance -- the same as mass in both common and scientific usage. In common usage, weight still means the same as mass.
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.
Mass is measured using a balance which compares an item in question to matter of known mass; this method is independent of gravity. Alternately, a spring scale or Hydraulic or pneumatic scale is used to measure force (which physicists call weight). Most scales measure weight using a spring.
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 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 center 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 not 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:
Human weight in the medical sciences and ordinary language
Although many people prefer the less-ambiguous term 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 0.45 kg). Many people in the United Kingdom still measure their weight using the stone equal to 14 lb (6.35 kg).
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.