Mass

Mass, in physics, measure of the linear inertia of a body, i.e., of the extent to which it resists acceleration when a force is applied to it. Alternatively, mass can be thought of as a measure of the amount of matter in a body. This view seems validated when one remembers that bodies of equal inertial mass have identical weights in a given gravitational field. The exact equivalence of inertial mass and gravitational mass is only a theoretical assumption, albeit one strongly supported by experimental evidence. According to Einstein's theory of relativity, the mass of a body is increased if it gains energy, according to the equation E=mc², where m is the change in mass due to the energy change E, and c is the electromagnetic constant. It is an important property of nature that in an isolated system mass-energy is conserved. The international standard of mass is the international prototype kilogram.

See also: Inertia.