| the principle that in any closed system subjected to no external forces, the mass is constant irrespective of its changes in form; the principle that matter cannot be created or destroyed. |
| conservation of mass
A principle of classical physics stating that the total mass of a closed system is unchanged by interaction of its parts. The principle does not hold under Special Relativity, since mass and energy can be converted into one another. |
conservation of matter
principle that the mass of an object or collection of objects never changes, no matter how the constituent parts rearrange themselves. Mass has been viewed in physics in two compatible ways. On the one hand, it is seen as a measure of inertia, the opposition that free bodies offer to forces: trucks are harder to move and to stop than less massive cars. On the other hand, mass is seen as giving rise to gravitational force, which accounts for the weight of an object: trucks are heavier than cars. The two views of mass are generally considered equivalent. Thus, from the perspective of either inertial mass or gravitational mass, according to the principle of mass conservation, different measurements of the mass of an object taken under various circumstances should always be the same
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