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== Mass–energy equivalence ==
{{Main|Mass-energy equivalence}}
As we increase an object's energy by accelerating it, such that its speed approaches the speed of light from an observer's point of view, its total (relativistic) mass increases, thereby making it more and more difficult to accelerate it from within the observer's frame of reference. This ultimately leads to the concept of mass-energy equivalence.
Any object that has mass when at rest (in a given inertial frame of refernece), equivalently has ''rest energy'' as can be calculated using Einstein's equation ''E''=''mc''<sup>2</sup>. Rest energy, being a form of energy, is interconvertible with [[Forms of energy|other forms]] of energy. As with any energy transformation, the total amount of energy [[conservation of energy|does not increase or decrease]] in such a process. From this perspective, the amount of matter in the [[universe]] contributes to its total energy.
Similarly, the total of amount of energy of any system also manifests as an equivalent total amount of mass, not limited to the case of the relativistic mass of a moving body. For example, adding 25 [[kilowatt-hours]] (90 [[megajoule|megajoules]]) of ''any form(s)'' of energy to an object increases its mass by 1 [[microgram]]. If you had a sensitive enough mass [[Weighing scale|balance or scale]], this mass increase could be measured. Our [[Sun]] (or a nuclear bomb) converts [[nuclear potential energy]] to other forms of energy; its total mass doesn't decrease due to that in itself because it still contains the same total energy in different forms, but its mass does decrease when the energy escapes out to its surroundings, largely as [[radiant energy]].
==Applications==
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