PURPOSE: To demonstrate the inertia of motion.

DESCRIPTION: A 22 kg. hunk of "dry ice" (solid carbon dioxide) will skate almost frictionlessly over a sheet of thick plate glass. The evolving vapor from the CO₂ invokes the so-called Leydenfrost effect, in which rapid production of vapor creates a continuously renewed gas cushion, which supports even heavy weights with near-zero friction. The impression of inertia given by the big hunk of CO₂, when you try to launch it suddenly, is overwhelming. Students can't blame the force required on friction. Consider: if the block is accelerated to just 1 meter per sec per sec, the force is 22 newtons -- over 5 pounds. Caution: students must handle the dry ice with gloves; its surface is about 110 degrees below zero F.

SETUP TIME: Once the dry ice is delivered, minimal.

COMPONENT LIST:

 Dry Ice

 Plate glass or other smooth surface.

SUGGESTIONS: See Vincent's 2122 Demonstration Booklet for further instructions.

REFERENCES: PIRA 1F30.13

OTHER NOTES:

(1) There may come a time in the demo when the hunk of CO₂ resting on the glass gives off a "motorboating" sound, and the hunk vibrates visibly. This is probably the "Trevelyan rocker" effect, which is too advanced to be usefully discussed at this level.

(2) The Leydenfrost effect is often cited as the "secret" of firewalking (walking on red-hot coals with bare feet). Some scientists endorse this and some don't.