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# mass-energy equivalence

- An equation derived from Einstein's theory of Special Relativity expressing the relationship between the mass and energy of objects with mass. The equation is
*E = mc*, where^{2}*E*is the energy of the object in joules,*m*is its**relativistic mass**in kilograms, and*c*is the speed of light (approximately 3 × 10^{8}meters per second). Mass-energy equivalence entails that the total mass of a system may change, although the total energy and momentum remain constant; for example, the collision of an electron and a proton annihilates the mass of both particles, but creates energy in the form of photons. The discovery of mass-energy equivalence was essential to the development of theories of atomic**fission**and**fusion**reactions.

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