1. The problem statement, all variables and given/known data "In frame of reference S, an electron moving along the x-axis has energy 3mc2 and momentum magnitude √(8)mc Use the transformations of energy and momentum to find the energy and momentum magnitude observed in ￼frame S′ moving with speed 4c/5 relative to S in the positive x-direction." 2. Relevant equations How can the electron have a proper velocity of c? If relativistic velocity is dx/dτ = γdx/dt, and relativistic momentum is simply the mass time the relativistic velocity, how is m(γ(dx/dt)) = √8mc? 3. The attempt at a solution I solved the problem correctly, I am just unsure of a detail in the wording of the problem.