Quantitative measure of inertia , or the resistance of a body to a change in motion.
The greater the mass, the smaller is the change produced by an applied force. Unlike weight , the mass of an object remains constant regardless of its location. Thus, as a satellite moves away from the gravitational pull of the Earth, its weight decreases but its mass remains the same. In ordinary, classical chemical reactions, mass can be neither created nor destroyed. The sum of the masses of the reactants is always equal to the sum of the masses of the products. For example, the mass of wood and oxygen that disappears in combustion is equal to the mass of water vapour, carbon dioxide, smoke, and ash that appears. However, Albert Einstein 's special theory of relativity shows that mass and energy are equivalent, so mass can be converted into energy and vice versa. Mass is converted into energy in nuclear fusion and nuclear fission . In these instances, conservation of mass is seen as a special case of a more general conservation of mass-energy. See also critical mass .