1. The problem statement, all variables and given/known data A cube of mass m slides without friction at a speed vo. It undergoes a perfectly elastic collision with the bottom tip of a rod length d and mass 2m. The rod is pivoted about a frictionless axle through its center, and initially it hangs straight down and is at rest. What is the cube's velocity- both speed and direction after the collision? 2. Relevant equations Moment of inertia for rod pivoted about center I=(1/12)mr^2 3. The attempt at a solution I used conservation of energy Kcubeinitial = Krotational of rod + Kcubefinal I replaced angular velocity with v1/.5d .5mvo^2 = .5mvf^2 + .5(1/12)2md^2 * (v1/.5)^2 Then I used conservation of momentum: mvo= mvf + 2mv1 But substituting these two equations into each other leads to something I can't solve. Should I try angular momentum?