1. The problem statement, all variables and given/known data An electron collides with a particle with mass M at rest and scatters elastically through an angle θ (assume electron mass negligible). Show that the fraction of energy lost by the e- is: (Ee - Ee')/Ee = 1/[1+ Mc2/Ee(1-cosθ)] 2. Relevant equations Conservation of Energy: Ee + Mc2 = Ee' + EM Conservation of momentum: Pe = Pe' + PM E2 = P2c2 + M2c4 or for electron since mass negligible, E=Pc Previous parts of the questions required the rearrangements of these to get: PM2 = 1/c2[Ee2 +Ee'2 - 2EeEe'cosθ] 3. The attempt at a solution I've tried to solve this so many times but the closest I can get is: (Ee - Ee')/Ee = Ee'(1-cosθ)/Mc2 I don't know if I'm missing some kind of relation I can use to sub in Ee' for Ee because in the final expression I'm trying to get there seems to be way more Ee's than I would expect. I've also tried working backwards to find out what I'm missing from the answer but I just don't see it. I think I have tunnel vision from trying this so often and can't see another way, so thank you in advance for any help, it's greatly appreciated!