1. The problem statement, all variables and given/known data a) The LHC was designed to collide protons together at 14 TeV centre-of-mass energy. How many kilometres per hour less than the speed of light are the protons moving? b) How fast is one proton moving relative to the other? 2. Relevant equations 3. The attempt at a solution a) We assume that the two beams are coming towards their common centre-of-mass with equal and opposite momenta, so that the centre-of-mass is at rest. In that case, if the centre-of-mass energy is 14 TeV, then the total energy of the system is 14 TeV. The centre-of-mass frame is the same as the laboratory frame, and so that the energy of each beam in the laboratory frame is 7 TeV. Am I correct so far?