The relationship of a transformation with regard to a particular kind of statistical view of order will be a crucial point of view we will learn later to apply to judge the transformations of energy. When you are applying this point of view, you are judging a transformation in terms of the total change in entropy of the system plus its surroundings. This is an vitally important framework for viewing physical change because it helps you to understand why some directions of change are spontaneous and some are not.

We will have a great deal more to say about this as this MCAT course progresses, but here, in the context of energy as a topic of mechanics, let it suffice to understand that some kinds of energy represent a more or less disordered state than others. Spontaneous change leads to an increase in the disorder (entropy) of the universe. This is the Second Law of Thermodynamics, one of our topics in a later module.

Most real world transformations are spontaneous, i.e. irreversible. In other words, it is much more likely for the sliding block to produce sound than for sound to make the block slide back up the plane.

The understanding that the entropy of the universe is always increasing is a very important concept governing the transformations of physical systems within their surroundings. When you move further into the physical sciences, you want to add to the first question: What is happening with energy? A second question: What is happening with entropy?

This is a preview of a very important set of concepts which we will approach with quite a bit of depth later in this course. We will have a great deal to say about entropy later. For now, keep in mind that not all directions of change are equal. Real world energy transformations are not reversible.