A box of total mass M is on a frictionless horizontal table. It is connected to a massless string that runs over a massless, frictionless pulley. A box of mass m is suspended from the other end of the string. The system is released from rest. Inside the box of mass M, a small ball is suspended from a string. Derive an expression for the angle θ the string makes with the vertical, in terms of M and m. (Hint: you should first find the acceleration of the system.)