1. The problem statement, all variables and given/known data What is the momentum of a proton with 800MeV of kinetic energy? 2. Relevant equations E^2=p^2c^2+m^2c^4 3. The attempt at a solution I know that E should be kinetic energy + rest energy, so I calculate rest energy first. rest energy = mc^2= (1.67*10^-27)*(3*10^8)^2 = 939.57 MeV. total E=939.57+800=1739.57 MeV Plug and chug and solve for p. m^2c^4=.1414MeV (this seems wrong) (1739.57)^2-.1414 = 3026103.644 MeV, divide that by (3*10^8)^2 and take the root to get p, which comes out to be 5.7985*10^-6. My answer needs to be in GeV, so it seems that I'm off by a factor of 40. I've been doing physics homework all day so my brain is fried and I've been beating my head against a wall for the last hour trying to solve this problem. I know that I'm missing something stupid, but I just can't see what it is. Help!