일반물리학 1 — Potential Energy and Conservation of Energy: Problem 20

20 A pendulum consists of a 2.0 kg stone swinging on a 4.5 m string of negligible mass. The stone has a speed of 8.0 m/s when it passes its lowest point. (a) What is the speed when the string is at \(60^\circ\) to the vertical? (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?