1. The problem statement, all variables and given/known data A particle moving with initial velocity vi = (3i + 5j) m-s−1, collides with a smooth plane wall placed at some orientation to the particle’s trajectory. The resulting velocity of the particle is vf = (−2i − j) m-s−1. The coefficient of restitution for this collision is Ans: 16/45 2. Relevant equations e = velocity of separation/velocity of approach 3. The attempt at a solution So, my first attempt was e = [ 0 - (−2i − j) ]/ [ (3i + 5j) - 0 ] = (2i + j)/(3i +5j) I thought of taking the speeds then, which gave me e = √5 / √34 Which is obviously not the answer. Then I tried to google COR, and i got-- "representing the ratio of speeds after and before an impact, taken along the line of the impact" I have a feeling that my answer is wrong because I'm not incorporating "line of impact". But I have no idea how to, because I'm not given the angle at which the particle collides with the wall.