A homogeneous bar of length L can rotate on a horizontal frictionless plane around a vertical axis passing during one of its ends (moment of inertia I = ML2/3). It is at rest until a small bullet of mass m with velocity v perpendicular to the bar hits it in the midpoint, bouncing back with velocity v’ = -v /3. Given that the mass M of the bar is 9 times larger than the mass m of the bullet, compute the final kinetic energies of the bullet and of the bar. Decide if the collision was elastic or not.