A pendulum consists of a 2.0 kg stone swinging on a 3.9 m string of negligible mass. The stone has a speed of 7.2 m/s when it passes its lowest point. a) What is the speed when the string is at 60° to the vertical? (in m/s) b) What is the greatest angle with the vertical that the string will reach during the stone's motion? c) If the potential energy of the pendulum-Earth system is taken to be zero at the stone's lowest point, what is the total mechanical energy of the system? (in Joules)