1. The problem statement, all variables and given/known data Mass of particle = 5GeV/C^2 Momentum of particle = 5GeV/c Find total energy of particle and speed of particle. 2. Relevant equations E=λmc^2 p=λmu λ=(√(1-(u/c)^2))^-1 3. The attempt at a solution When I try so solve for speed, I realize λ is a function of u, so get this: (u*(√(1-(u/c)^2))^-1) = p/m = c, but can't solve for u from that. How would I go about approaching this problem?