A demonstration of physics—how an object moves through fluid with the effects of friction and dampening showing. It's based on the extraordinary ideas of Keith Peters, as well as an implementation of one those ideas by Hamza Fawzi. These are two smart people.

Because Professional Writing is about placing the reader at the center, I've added a way for users to directly vary the two key variables of speed and drag through the fluid.. Fawzi's original version let users only pull the "snake" (his term) around the screen (a very different meaning of drag).

This relatively simple change powerfully increases user involvement. The original appeal of the elastic snake remains—it's fun to pull around the screen—but the adding user control of variables produces an exponential rise the number of possible behaviors of the balls. That in turn makes the reader much more active and involved. In this version, user control variables vianumeric input; another version controls variables via a slider.

This has obvious pedagogical implications for teaching motion physics, but my goal here is to demonstrate a purely professional writing maxim: Give the reader control—and show the consequences of ignoring the suggested range of values to alter.