1. The problem statement, all variables and given/known data I have a relativistic electron with energy 10^10 eV and I want to work out its speed. I'm not told if that's total energy or just kinetic energy - which is the sensible assumption to make? 2. Relevant equations (E_tot)^2 = p^2*c^2 + m0^2*c^4 p = gamma*mv gamma = sqrt(1/1-beta^2) beta = v/c 3. The attempt at a solution I tried rearranging the above equations to get an expression for v, but I keep getting stupid answers (v = c, v > c). I have converted the energy into J and assumed that it is the total energy. Can anyone tell me what I'm doing wrong? Thanks!