What it shows: Two bodies, rotating about each other, rotate about their common center-of-mass (COM). The COM exhibits uniform motion (or none at all) regardless of what the two bodies are doing.
How it works: The "bodies" are 4-1/2" diameter acrylic disks that float on a cushion of air on a large air table.1 Presently we have three versions ready to go. (1) The first version has two disks connected by means of a 12"- long plastic ruler. A large "dot" at the center of the ruler marks the COM. The disks can be made to simply spin about their COM, or traverse in pure translational motion, or a combination of the two. (2) The second version consists of three disks; two are stacked together and the stack is connected to the third. In this case the COM is located at the 4" mark on the ruler. (3) The third version is meant to model a diatomic molecule. Two disks are connected by means of a spring rather than a stiff ruler. The disks can undergo vibrational, rotational, and translational motion, or any combination. Regardless of the complicated disk motion, the COM motion is observed to be uniform.
Setting it up: The air table rests on a dedicated three-legged support. Attach two air hoses to the T-connector. If you find it difficult to get the hose adaptor onto the T-connector, warm the adaptor with a heat gun to soften it up. Air is supplied by two Air Sources.2 Crank up their outputs to maximum. Level the table dynamically by adjusting the three leveling screws so that a disk remains in the center. For larger audiences, you may wish to use video projection with a camera shot from above the table. If you have a generous amount of set-up time, it's especially effective to illuminate the table with UV "black" lights. Use a fluorescent COM dot and turn down the overhead lights.
1. Ealing-DAW 4-ft square air table
2. Pasco SF-9216 Air Supply