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A quantitative measure of the resistance of an object to attempts to change its velocity. Mass is often defined as Hence mass is a measure of inertia (from the Latin vis inertia, the force of inaction.) |
Newton equated mass and "quantity of matter in a body. _Gravitational Mass_", which he said arises "from its density and bulk conjointly."
Gravitational Mass, which appears in the Law of Gravitation, is identical to this _Inertial Mass_ . A proper explanation of this involves the _Principle of Equivalence_ and _General Relativity_, and is beyond the scope of this introductory wiki. The interested student is directed toward books on relativity, or to look up these terms on the internet gravitation (universal), has been shown to be proportional to Inertial Mass to better than one part per trillion. [See the Eotvos experiment page in Wikipedia, much more accurately performed at the University of Washington - Physical Review Letters 100, 041101 (2008).] This equality is assumed by the Principle of Equivalence and is a cornerstone of Einstein's theory of General Relativity. This theory indicates that all forms of energy have mass via the relationship E=mc2.