Mass (physics)

Mass (physics), amount of matter that a body contains, and a measure of the body's inertia, that is, of its resistance to change of motion. Mass is different from weight, which is a measure of the attraction of the Earth for a given mass (see Gravitation). Inertial mass and gravitational mass are identical. Weight, although proportional to mass, varies with the position of a given mass relative to the Earth; equal masses at the same location in a gravitational field will have equal weights. A fundamental principle of classical physics is the law of conservation of mass, which states that matter cannot be created or destroyed. This law holds true in chemical reactions but is modified in cases where atoms disintegrate and matter is converted to energy or energy is converted to matter (see Nuclear Energy; X-Ray: Pair Production).

The theory of relativity, initially formulated in 1905 by Albert Einstein, did much to change traditional concepts of mass. Relativity shows that the mass of an object changes as its velocity approaches that of light, that is, when it approaches 300,000 km/sec (about 186,000 mi/sec); an object moving at a speed of 260,000 km/sec (about 160,000 mi/sec), for example, has a mass about double its so-called rest mass. When bodies have such velocities, as particles produced in nuclear reactions do, mass can be converted into energy and vice versa, as suggested by Einstein in his famous equation E = mc² (energy equals mass multiplied by the velocity of light squared).

See also International System of Units; Mechanics; Quantum Theory.