1. The problem statement, all variables and given/known data A billiard ball (Ball #1) moving at 5.00 m/s strikes a stationary ball (Ball #2) of the same mass. After the collision, Ball #1 moves at a speed of 4.35 m/s. Find the speed of Ball #2 after the collision. 2. Relevant equations Conservation of Kinetic Energy Conservation of Momentum 3. The attempt at a solution I used the Pi = Pf equation of conservation of momentum and got 0.65 m/s while the conservation of Kinetic Energy equation yielded the correct answer: 2.47 m/s. What's wrong? Why couldn't I use the momentum equation too?