In one class, I finally said why I called this unit the mechanical wave model, since mechanical waves cannot use a vacuum as its medium. (I always mention that a vacuum is not a vacuum cleaner, as I say every time, since I have had students not understand why I kept talking about rug care in my class.) So we did a few old AP problems in the few minutes we had before a spirit week assembly, and I felt it helped students solidify their understanding.
