A stone of 20 kg of mass is held 15 m above a tank containing 200 kg of water. The stone, water and the air are at the same temperature initially. The stone then falls into the water. Choosing the stone as the system, and starting from a statement of the first law of thermodynamics, determine, ?U, ?PE, ?KE, Q and W, (magnitude and sign), when each of the following states is reached:
a. The stone is about to enter the water,
b. The stone has come to rest in the tank, (assume it decelerates instantly, as soon as it hits the water), and
c. When the stone and water return to their initial temperature.