The higher the hill the more potential energy the rock has. After the rock smashes Willey, it has no potential energy left. There is a subtle difference between work and energy. This is just the part of the universe that we are paying attention to. In the Willey E. Coyote example the system is the rock and I can describe the energy of the rock.
Thermochemistry is about measuring the energy difference between the initial and final states.
Heat and work. The energy of a system is divided into heat and work. As the rock falls, the system loses energy so Delta Energy is negative. This section introduces some new terms. It is important to keep in mind that we can only measure change.