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Weight is a term of measurement referring to either an object's mass or to the gravitational force acting on the object. Its exact meaning depends on the context in which it is used. The difference between these quantities is, historically, a relatively recent innovation, and the term "weight" continues to serve both purposes today. In the physical sciences, it is often narrowly defined to refer only to the force due to gravity, while in everyday discourse it is often used synonymously with mass. However, it should be emphasized that mass and gravitational force are quite dissimilar properties of a given object, despite being proportional to each other when the object is subject to non-zero gravitational acceleration.
Usage In scientific usage in the field of mechanics, weight and mass are quite different quantities: mass is an intrinsic property of matter, whereas weight is a force that results from the action of gravity on matter. However, confusion between these definitions abound, even in scientific terminology. For instance, the chemical terms "atomic weight", "molecular weight", and "formula weight" are still common, but should be discouraged in favour of more precise terminology such as atomic mass and so on. In everyday usage, however, the terms "weight" and "mass" are usually not different. For instance, when goods are bought and sold "by weight", the amount of goods exchanged (the mass) is the quantity of interest, not how hard they press down on the table (the force). Similarly, measurements of body weight focus on the amount of tissue (fat, muscle, etc.) present. Although it would be more accurate (and therefore preferable) to say that an object "masses one kilogram," such phrasing remains uncommon, and is considered equivalent to saying that an object "weighs one kilogram." The gravitational force exerted on an object is directly proportional to its mass, so a mass of x kilograms always weighs x times as much as a mass of one kilogram in a given gravitational field. Because of this proportionality, it is not necessary to distinguish between mass or force in most contexts. The distinction between mass and force due to gravity becomes obvious when comparing objects in various gravitational fields, such as away from the earth's surface. For example, on the surface of the Moon, gravity is only about one-sixth as strong as on the surface of the earth. A one-kilogram mass is still a one-kilogram mass (as mass is an intrinsic property of the object) but the downwards force due to gravity is only one-sixth of what the object would experience at the surface of the earth. For this reason, weight refers specifically to the force rather than the mass in this context. Units of weight See also: force or mass Systems of units of force and mass have a tangled history, partly because the distinction was not properly understood when many of the units first came into use. If SI units such as the kilogram are used, it can be inferred that mass is intended, but if the quantity is given in pounds, the meaning may depend on context. The governments of many nations, including the United States, the United Kingdom, and others, have officially defined the pound as a unit of mass. Even so, the use of pounds to measure force is still common in engineering, and it occurs in units like psi. It is also common to disambiguate the meaning with compound terms like pound-force. In modern scientific work, the units of weight are simply units of force. The SI unit of force is the newton (N), which can also be expressed in SI base units as kg·m/s² (kilograms times metres per second squared). The kilogram-force is a non-SI unit of weight, defined as the force exerted by a one-kilogram mass in Earth gravity (about 9.8 newtons). In United States customary units, the pound can be used as a unit of force. Alternatively, the poundal has been used, defined as the force necessary to accelerate a one-pound object at 1 ft/s². A poundal is about 1/32 of a pound. The SI unit of mass is the kilogram. In US Customary Units, the pound can also be used as a unit of mass. Alternatively, the slug is used, defined as the amount of mass that accelerates at 1 ft/s² when a pound of force is exerted on it. A slug is about 32 pounds. To convert between units of force and mass, we need to know the strength of gravity. Usually this is the so-called standard gravity (approximately 9.8 m/s² or 32 ft/s²). Plugging this value into Newton's second law, F = ma (force = mass × acceleration), we see that a one-kilogram mass experiences a gravitational force of 1 kg × 9.8 m/s² = 9.8 newtons. In general, to convert mass in kilograms to force in newtons (at the earth's surface), multiply by 9.8. Conversely, to convert newtons to kilograms divide by 9.8. (32 is the conversion factor for US Customary Units). The gravitational force exerted on an object is proportional to the mass of the object, so it is reasonable to measure the strength of gravity in terms of force per unit mass, that is, newtons per kilogram (N/kg). However, the unit N/kg resolves to m/s²; (metres per second per second), which is the SI unit of acceleration, and in practice gravitational strength is usually quoted as an acceleration. Sensation of weight The weight force that we actually sense is not the downward force of gravity, but the normal (upward) force exerted by the surface we stand on, which opposes gravity and prevents us falling to the center of the Earth. This normal force, called the apparent weight, is the one that is measured by a spring scale. For a body supported in a stationary position, the normal force exactly balances the earth's gravitational force, and so apparent weight has the same magnitude as actual weight. (Technically, things are slightly more complicated. For example, due to the earth's rotation objects are subject to a small centrifugal force, varying with latitude, which partially offsets gravity. The normal force therefore balances a force slightly less than the true force of gravity.) If there is no contact with any surface to provide such an opposing force then there is no sensation of weight (no apparent weight). This happens in free-fall, as experienced by sky-divers and astronauts in orbit who feel "weightless" even though their bodies are still subject to the force of gravity. The experience of having no apparent weight is also known as microgravity. A degree of reduction of apparent weight occurs, for example, in elevators. In an elevator, a spring scale will register a decrease in a person's (apparent) weight as the elevator starts to accelerate downwards. This is because the opposing force of the elevator's floor decreases as it accelerates away underneath one's feet. See under Apparent weight for a more detailed explanation of this phenomenon. Measuring weight A spring scale or hydraulic or pneumatic scale measures the weight force (strictly the apparent weight force) directly. Most scales measure weight using a spring. Household scales that are calibrated in units of mass (such as kilograms) assume that standard gravity will apply. Mass may be measured with a balance, which compares the item in question to others of known mass. The force can then be inferred from the mass by multiplying by the acceleration of gravity, if it is known. Relative weights on the Earth, on the moon and other planets 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: Although gravity at the Earth's surface is 'nearly' constant, it does vary slightly with location, which means that objects have slightly different weights in different places. For further information see acceleration due to gravity, physical geodesy, gravity anomaly and gravity. Body weight 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, although in some countries people more often measure their body weight in pounds (e.g. USA) or stone and pounds (e.g. UK) and thus may not be well acquainted with measurement in kilograms. 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, and a stone (14 lb) is 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. See also | ||||||||
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