1. The problem statement, all variables and given/known data Two blocks collide on a perfectly elastic collision, after sliding on a frictionless surface. va = 1m/s vb = 1m/s ma = 0.05kg mb = 0.03kb I need to find the speed of both blocks after the crash. 2. Relevant equations Conservation of momentum: mava+mbvb = mava' + mbvb' Conservation of energy: 1/2mava^2 + 1/2mbvb^2 = 1/2mava'^2 + 1/2mbvb'^2 General solution for these two equations: -(vb−va)=vb'-va' Refer to this thread for cool formatting of these equations (i dont really know how to do that): https://www.physicsforums.com/threads/elastic-collision.753780/ I dont really understand where that last equation comes from or how to get there. 3. The attempt at a solution I solved for the conservation of momentum: mava+mbvb = mava' + mbvb' 0.05kg.1m/s + 0.03kg.1m/s = 0.05kg.va' + 0.03kg.vb' 0.08ms = 0.05va' + 0.03vb' (*) Then on the third equation: -(vb−va)=vb'-va' -(-1m/s - 1m/s) = vb'-va' 2m/s = vb'-va' 2m/s +va' = vb' Then i plug this on (*) and i get 0.08m/s = 0.05va' + 0.03(2m/s + va') 0.08m/s = 0.05va' + 0.06m/s + 0.03va' 0.02m/s = 0.08va' 0.25m/s = va' then vb' = 2.25m/s The expected solution is 0.5m/s and 2m/s . So my two questions are, am i doing something wrong here? And how do i get to the third equation?