[Suicide Junkie]

Electrons aren't massless...
Constant #03 on my calculator reports the accepted rest mass of the electron as 9.1093897 x10^-31 kg.

Very small, certainly, but still worth a half-million electron volts of energy.

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The e=mc² is the rest energy when you plug in the rest mass. If you fling the object, then the you have to add on the energy you just added.