How fast must an electron move in order for its mass in motion to be twice its rest mass?

The mass of the body “m” moving with the speed “V” is determined by the formula: m = m0 / square root of (1 – V ^ 2 / c ^ 2), where “m0” is the mass of the body at rest, “c” is the speed of light From the formula, it can be seen that a doubling of the mass is possible if the denominator of the formula becomes 0.5: m = m0: 0.5 = 2 * m0 Thus we get: square root of (1 – V ^ 2 / c ^ 2) = 0.5 1 – V ^ 2 / c ^ 2 = 0.5 ^ 2 1 – V ^ 2 / c ^ 2 = 0.25 V ^ 2 / c ^ 2 = 1 – 0.25 V ^ 2 / c ^ 2 = 0.75 V ^ 2 = 0.75 * c ^ 2 V = square root of (0.75 * c ^ 2) V ~ = 0, 87 * c I.e. the electron must accelerate to 0.87 of the speed of light or: V = 0.87 * c = 0.87 * 300’000 [km / s] ~ = 261’000 [km / s]
Answer. 261’000 km / sec.