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...

What it Shows

Inspired by Richard Feynman's story in his 1985 book (pp 63-65), Surely You're Joking Mr. Feynman, the demonstration answers the question "which direction does a lawn sprinkler spin if water enters the nozzle rather than being expelled from the nozzle?" The reverse sprinkler spins in the opposite direction of a "normal" sprinkler. "Dissipative effects" has been the hand-waving reason for the past 30 years, but the real reason why it spins in the reverse direction is far from obvious (see Comments, below). It turns out that a sprinkler designed to be "truly...

What it shows:

The electroscope, or electrometer, is an instrument that measures electric charge or voltage by means of repulsive electrostatic force.

Sodium undergoes a reaction with water.

A liter of warm water in large pyrex vessel, covered with fine mesh stainless steel screen, is on a stool close by in-floor vent hood.  Add a few drops from the phenolphthalein indicator bottle.

Using the long forceps, pick out a pea size lump of sodium metal from the mineral oil in the small beaker. Wipe off the lump on the dry paper towels. With the vent fan running, lift the edge of the screen and drop in the sodium metal. Replace the screen and get back.

The sodium will from a hissing ball of molten metal, which bounces...