Mass (physics), in physics, amount of matter that a body contains, and a measure of the inertial property of that body, that is, of its resistance to change of motion (see Inertia). 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; thus, equal masses at the same location in a gravitational field will have equal weights. A mass in interstellar space may have nearly zero weight. 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;

The theory of relativity, initially formulated in 1905 by the German-born American physicist Albert Einstein, did much to change traditional concepts of mass. In modern physics, the mass of an object is regarded as changing 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 approximately 260,000 km/sec (about 160,000 mi/sec), for example, has a mass about double its so-called rest mass. Where such velocities are involved, as in nuclear reactions, 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

**X Ray**).The theory of relativity, initially formulated in 1905 by the German-born American physicist Albert Einstein, did much to change traditional concepts of mass. In modern physics, the mass of an object is regarded as changing 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 approximately 260,000 km/sec (about 160,000 mi/sec), for example, has a mass about double its so-called rest mass. Where such velocities are involved, as in nuclear reactions, 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.