weight, measure of the force of gravity on a body (see gravitation). Since the weights of different bodies at the same location are proportional to their masses, weight is often used as a measure of mass. However, the two are not the same; mass is a measure of the amount of matter present in a body and thus has the same value at different locations, and weight varies depending upon the location of the body in the earth's gravitational field (or the gravitational field of some other astronomical body). A given body will have the same mass on the earth and on the moon, but its weight on the moon will be only about 16% of the weight as measured on the earth. The distinction between weight and mass is further confused by the use of the same units to measure both—the pound, the gram, or the kilogram. One pound of weight, or force, is the force necessary at a given location to accelerate a one-pound mass at a rate equal to the acceleration of gravity at that location (about 32 ft per sec per sec). Similar relationships hold between the gram of force and the gram of mass and between the kilogram of force and the kilogram of mass.

System of conditioning involving lifting weights, especially for strength and endurance. It may include the use of barbells and dumbbells, a Nautilus or similar machines, or a combination of these. Athletes use it to improve their performance, nonathletes use it for general conditioning or bodybuilding, and those recovering from an injury may use it as part of an overall rehabilitation program.

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Sport in which barbells are lifted competitively or as an exercise. The two main events are (1) the snatch, in which the barbell is lifted from the floor to arm's length overhead in a single, continuous motion; and (2) the clean and jerk, in which it is lifted first to the shoulders and then, after a pause, to arm's length overhead. Contestants are divided into 10 body-weight categories ranging from flyweight to superheavyweight. Lifts may range to over 1,000 lbs (455 kg) in the heavyweight divisions. The origins of modern competition are to be found in 18th- and 19th-century strongman contests. The first three Olympic Games (1896, 1900, 1904) included weight lifting, as have all games after 1920.

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Gravitational force of attraction on an object, caused by the presence of a massive second object, such as the Earth or Moon. It is a consequence of Isaac Newton's universal law of gravitation, which states that the force of attraction between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. For this reason, objects of greater mass weigh more on the surface of the Earth. On the other hand, an object's weight on the Moon is about one-sixth of its weight on Earth, even though its mass remains the same, because the Moon has less mass and a smaller radius than the Earth and therefore exerts less gravitational force. Weight math.W is the product of an object's mass math.m and the acceleration of gravity math.g at the location of the object, or math.W = math.mmath.g. Since weight is a measure of force rather than mass, the units of weight in the International System of Units are newtons (N). In common usage, weight is measured by the gram in the metric system and by the ounce and pound in the U.S. and British systems.

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Sum of the atomic weights of all atoms in a chemical formula. The term is generally applied to a substance that consists of ions (see ionic bond) rather than individual molecules (and thus does not have a molecular weight). An example of such a substance is sodium chloride (table salt). Such a substance's chemical formula describes the simplest ratio of the number of atoms of the constituent elements. Seealso stoichiometry.

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or combining weight

Quantity of an element that exactly reacts with (equals the combining value of) 1 g of hydrogen, 8 g of oxygen, or a corresponding amount of any other element. An element's equivalent weight is its atomic weight divided by its valence. In general, for oxidation-reduction, including electrolysis, the equivalent weight is the weight associated with the loss or gain of 6.02 × 1023 electrons (Avogadro's number) or 96,500 coulombs of electric charge; this is also the molecular weight divided by the number of electrons lost or gained. The equivalent weight of a substance with several valences differs depending on the number of electrons transferred in the given reaction. The number of equivalent weights of any substance dissolved in one litre of solution is called the solution's normality (math.N). Seealso stoichiometry.

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Ratio of the average mass of a chemical element's atoms to 112 the mass of an atom of the carbon-12 isotope. The original standard of atomic weight, established in the 19th century, was hydrogen, with a value of 1. From circa 1900 until 1961, the reference standard was oxygen, with a value of 16, and the unit of atomic mass was defined as 116 the mass of an oxygen atom. Oxygen, however, contains small amounts of two isotopes that are heavier than the most abundant one, and 16 is actually a weighted average of the masses of the three isotopes of oxygen. Therefore, the standard was changed to one based on carbon-12. The new scale required only minimal changes to the values that had been used for chemical atomic weights.

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