At what energy does an electron become “relativistic”? Think electrons with kinetic energies of 50 eV, 50 keV, and 50 MeV. For each case, compute the momentum of the electron first using the non-relativistic formula for kinetic energy, and then using the correct relativistic formulas. Express the momentum in units of eV/c, or MeV/c or keV/c (whichever is appropriate), as discussed in section 2.13 of Thornton and Rex. (For this you need to know that the rest energy of an electron is 0.511 MeV.) Contrast your answers for each case. When is it important to use the relativistic formulas?