What is the mass in mev and in kg (both to decimal places) of an electron whose speed is half the speed of light? mass in mey- mass in kg- b. what is the momentum of this electron, in kg. s-. and in mev (both in 3 decimal places)? momentum of this electron, in kg·s-1= momentum in mev/c c. what would be the energy (in ey) of a photon with the same speed energy of photon in ev-