1. The problem statement, all variables and given/known data A particle of rest mass M0 is at rest in the laboratory when it decays into three identical particles, each of rest mass m. Two of the particles have velocity u1=-4/5c i(vector) u2=-3/5c j(vector) Calculate the direction and speed of particle 3 2. Relevant equations pf-pi=0 p=(gamma)mu 3. The attempt at a solution The starting momentum = 0 since it is at rest, so the momentum of the three particles should add up to 0. p1=m/(sqrt(1-(-4/5)^2)) -i(vector) p2=m/(sqrt(1-(-3/5)^2)) -j(vector) p3=m/(sqrt(1-(v/c)^2)) ij(vector) the total momentum is p1+p2+p3 I calculated m/.6 -i + m/.8 -j + m/(sqrt(1-v/c)^2)) ij I'm not sure how to break this up to find v. I had tried to use u=(sqrt(ux^2+uy^2+uz^2)) but that = 1 for this. I think to use that equation I need to break down the ij vector of particle 3 To do this it should be atanx=(3/4), but the answer is actually atanx=(9/16) I'm not sure why it's squared. Likely I'm forgetting something very basic that messed up the whole problem. Main points of interest: find out i and j vectors of particle 3 find out why the units are squared as in atanx=(9/16) and is that 9/25 (3/5)^2? if so that doesn't compute out using sin 29 = (9/25)/h M/m should be (gamma)1*m+(gamma)2*m+(gamma)3*m since they all have different gammas.