Trebuchet rocks at Caerlaverock Castle

This sandstone trebuchet rock at Caerlaverock has a diameter of about 24cm and thus a radius of 12cm. Hence volume is about 7000 cubic centimetres. The density of sandstone is about 2.3 grams per cubic centimetre so the mass of the rock is about 16kg. I was wondering about dealing with the energy flow by using the First Law system boundaries and having either heat or work flow over the boundary. Could the boundary be drawn around the ball itself? If so, I can see work flowing in when the ball is fired, but is it possible to do this with a solid instead of a gas, and dealing with bulk energies like kinetic rather than internal energy? More thought required.