The quarks that make the proton are up, up and down with masses of around 2, 2 and 4 MeV/c^2 respectively. So, how come when added them together we arrive at a particle with the huge mass of about 931 MeV/c^2. The only reason I can think of to explain this mass difference is the mass equivalence of the binding energy of the 3 quarks, however, I'm a little hesitant of this explanation because the mass difference is so large. I've looked around on the internet and in a couple of particle physics books but I can't find any information. Thanks for your help.