The quantity of matter in a body. The inertial mass is the mass as a measure of resistance of a body to changes in its motion, and the gravitational mass is the mass as a measure of the attraction of one body to another. The general theory of relativity shows inertial and gravitational mass are equivalent. By a suitable choice of units they can be made numerically equal for a given body, which has been confirmed to a high degree of precision by experiment. Mass is always conserved. See mass-energy equation, relativistic mass equation, rest mass,

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