Mass
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 ; X Ray ). The theory of relativity , initially formulated in 1905 by the Germa