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Einstein's mass - energy relation emerging out of his famous theory of relativity relates mass (m) to energy (E ) as E = mc^2, where c is speed of light in vacuum. At the nuclear level, the magnitudes of energy are vary small. The energy at nuclear level is usually measured in MeV, where 1 MeV = 1.6 xx 10^(-13) J , the masses are measured in unified mass unit (u) where 1 u = 1.67xx10^(-27)kg. (a) Show that the energy equivalent of 1u is 931.5 MeV. (b) A student writes the relation as 1 u = 931.5 MeV. The teacher points out that the relation is dimensionally incorrect. Write the correct relation.

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Answer Text

Solution :
(a) When `m = 1 u =1.67xx10^(-27)kg` <br> `E = mc^2 = 1.67xx10^(-27)(3xx10^8)^2 J= 1.67xx9xx10^(-11)J` <br> `E = (1.67xx9xx10^(-11))/(1.6xx10^(-13)) MeV = 939.4 MeV` <br> (b) The dimensionally correct relation is `1 u xx c^2 = 939.4 MeV`

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