Hello guys, The question is: What is the momentum, in conventional SI units, of a proton of momentum 685 MeVc-1? So, I tried two methods which yielded slightly different answers, both proximate to the actual answer (3.66 x 10-19) What I first tried to do was to rearrange: ρ=γm0v To find the particles velocity and then multiply it by the proton's rest mass in kg. I got 2.995 x 10-19 kgms-1 The second method, which gave me the result 3.45 x 10-19 kgms-1 was the following: From: E2 = (m0)2c4 + p2c2 I got a value for total energy. I then subtracted the rest energy from that and I got a value for KE. I equalled that to p2/2m and solved for p (using m in kg and the energy converted from MeV to J) Are my answers wrong? The book says 3.66 x 10-19 and I simply can't get that! Thanks in advance!