1. The problem statement, all variables and given/known data At the LHC at CERN protons with an energy of 6.5 TeV (= 6.5·1012eV) each are collided with each other.To achieve the same invariant mass in a ﬁxed target experiment, what would the energy of the proton beam have to be? 2. Relevant equations E2 - p2c2 = m2c4 E2 - p2c2= (m2c2-m2v2)/(1-v2/c2) 3. The attempt at a solution Because the protons are colliding p =0. From there I used the first equation to solve for m. m = .00014. This is my invariant mass, right? Now, with the proton colliding with a wall, there is momentum to be considered, right? This is where I get stuck. I need to know the velocity of the protons to find their momentum, don't I? I thought I could us E = (mc2)/√(1-v2/c2) and solve for v but it doesn't work out. Any feedback on where I'm not understanding something correctly or missing something would be helpful.