4. A steel ball of mass m is fastened to a light cord of length L and released when the cord is horizontal. At the
bottom of its path, the ball strikes a hard plastic block of mass M = 3m, initially at rest on a frictionless
surface. The collision is elastic.
(a) Find the tension in the cord when the ball’s height above its lowest position is L/3. Write your answer
in terms of m and g.
(b) Find the speed of the block immediately after the collision.
(c) To what height h will the ball rebound after the collision?