So far an optional section. At this point it represents something of a throwback to the discussions of ladders against walls, but we are also set up to work through the example and include it en passant as a tool, if desired. The particular example of relevance here would be Galilleo's ball on the plane, because in addition to being the archetype of this sort of problem, it allows us a point of historical accuracy, since this was the actual way Galilleo arrived at the conclusion that properties of the object don't determine how it falls.