1. The problem statement, all variables and given/known data Two equal masses, each m, are resting at the ends of a uniform rod of length 2a and negligible mass. The system is in equilibrium about the center C of the rod. A piece of clay of mass m is dropped down on the mass at the right end, hits it with velocity v as shown below and sticks to it. The ratio of the kinetic energy Ef just after the collision to the kinetic energy Ei just before the collision, Ef/Ei, would be: (A) 1 (B) 3/4 (C) 2/3 (D) 1/2 (E) 1/3 2. Relevant equations Not really sure. 3. The attempt at a solution If the system is initially in equilibrium, then it's not moving, and I don't understand how it could have kinetic energy if it's not moving. I want to write: Ef/0, but that's obviously wrong.