1. The problem statement, all variables and given/known data During a game of billiards, a white cue ball traveling at speed v strikes a green ball that was initially at rest. The green ball's speed after the collision is twice the speed of the white ball after the collision. The billiard balls have equal mass. What is the angle between the cue ball's final velocity vector and the cue ball's initial velocity vector? (Enter an angle between 0 and 90 degrees.) 2. Relevant equations v^2 = v_1^2+v_2^2 v = v_1+v_2 3. The attempt at a solution I have determined that the speed of the cue ball after hitting the green ball is v/sqrt(5) making the speed of the green ball 2v/sqrt(5). From what I can tell cos(theta) = speed of cue ball after striking green ball/initial speed = v/sqrt(5)/v = 1/sqrt(5). But obviously this isn't correct. What am I doing wrong?