We introduced energy and its conservation in the last chapter. The key point to reiterate here is that we can apply the principle of conservation of energy to work out how things move and how they interact with other things.
It’s also important to remember that the principle can apply to the conversion of energy between different forms, as well as to the transfer of energy between different objects. Here is a statement of the principle once more, in a slightly different form:
In any conversion of energy between different forms, or in transfer of energy between different objects, the total amounts of energy before and after the conversion or transfer are always equal.
A particularly useful form of the principle ...