Mass is quantitative measure of inertia, a fundamental property of all matter. It is, in effect, the resistance that a body of matter offers to a change in its speed or position upon the application of a force. The greater the mass of a body, the smaller the change produced by an applied force. Although mass is defined in terms of inertia, it is conventionally expressed as weight. By international agreement the standard unit of mass, with which the masses of all other objects are compared, is a platinum-iridium cylinder of one kilogram. This unit is commonly called the International Prototype Kilogram and is kept at the International Bureau of Weights and Measures in Shvres, Fr. In countries that continue to favour the English system of measurement over the International System of Units (SI), the current version of the metric system, the avoirdupois pound is used instead. Another unit of mass, one that is widely employed by engineers, is the slug, which equals 32.17 pounds.

Weight, though related to mass, nonetheless differs from the latter. Weight essentially constitutes the force exerted on matter by the gravitational attraction of the Earth, and so it varies from place to place. In contrast, mass remains constant regardless of its location under ordinary circumstances. A satellite launched into space, for example, weighs increasingly less the further it travels away from the Earth. Its mass, however, stays the same.

For years it was assumed that the mass of a body always remained invariable. This notion, expressed as the theory of conservation of mass, held that the mass of an object or collection of objects never changes, no matter how the constituent parts rearrange themselves. If a body split into pieces, it was thought that the mass divided with the pieces, so that the sum of the masses of the individual pieces would be equal to the original mass. Or, if particles were joined together, it was thought that the mass of the composite would be equal to the sum of the masses of the constituent particles. But this is not true.

With the advent of the special theory of relativity by Einstein in 1905, the notion of mass underwent a radical revision. Mass lost its absoluteness. The mass of an object was seen to be equivalent to energy, to be interconvertible with energy, and to increase significantly at exceedingly high speeds near that of light (about 3 108 metres per second, or 186,000 miles per second). The total energy of an object was understood to comprise its rest mass as well as its increase of mass caused by high speed. The mass of an atomic nucleus was discovered to be measurably smaller than the sum of the masses of its constituent neutrons and protons. Mass was no longer considered constant, or unchangeable. In both chemical and nuclear reactions, some conversion between mass and energy occurs, so that the products generally have smaller or greater mass than the reactants. The difference in mass is so slight for ordinary chemical reactions that mass conservation may be invoked as a practical principle for predicting the mass of products. Mass conservation is invalid, however, for the behaviour of masses actively involved in nuclear reactors, in particle accelerators, and in the thermonuclear reactions in the Sun and stars. The new conservation principle is the conservation of mass-energy.

Excerpt from the Encyclopedia Britannica without permission.