I don't think it was in my course book!-III

Find the speed at which the mass of an electron is double of its rest mass.

Assume that the mass of electron equals to $$9.1 \times 10^{-31} \text{kg}$$, $$c = 3 \times 10^8 \text{ m/s}$$.

Hint: Use the equation for relativistic mass given by Lorentz transformation.

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