1. The problem statement, all variables and given/known data a steel ball of m mass is fastened to light cord of lenght L and released when the cord is horizontal. At the bottom if its path the ball strikes a hard plastic block of mass M=4m, initially at rest on a frictionless surface. The collision is elastic. Find the speed of the block immediatey after the collision. 2. Relevant equations KE+U+W=KEF+UF m1v1+m2v2=m1v3+m2v4 3. The attempt at a solution I get the as speed (v1+4v2-v3)/4 but the book gets 2/5√2gL If anyone can help it would be much appreciated.