1. The problem statement, all variables and given/known data A solid sphere is rolling without slipping on rough ground with an angular velocity w and linear velocity v. It collides elastically with an another identical sphere at rest. Radius of each sphere is R and mass m. What is the linear velocity of the first sphere after it starts pure rolling again? 2. Relevant equations The equations for the conservation of angular momentum. 3. The attempt at a solution The basic idea is conserving angular momentum about the point of contact. Once that is done,the problem will be solved easily. Framing the equation for the conservation of momentum appears to be a problem (for me).