As far as teaching Newton's First law goes the most convincing demo for my students is
the personal shopvac powered hovercraft (construction has been detailed in many
sources). It is the one demo that truly shows the first law behaviors of linear motion
and constant speed. Any sized student can be moved by the smallest of forces and the
sight of a student gliding across the gym without a natural inclination to stop is
most memorable. I use the extension cord to whirl the student in a circle to create a
mini solar system and show that the Sun's gravity changes the direction of motion but
does not cause it. It is one of my best days in class.
Most other "inertia demos" (tablecloth pulling, hoop snatching, etc. ) almost do more
harm than good since they are demos of no (little) force not acting on an object
causing nothing to happen. Most students come away from those demos believing that
inertia "holds" the object there in spite of a force being applied - ugh! I still do
these demos but I discuss extensively that they are not "illustrative" of the first
law properties.
I have written a web based course for WebED <http://www.webed.com> on presenting
inertia demos if you are interested. (Please forgive the self promotion)
HTH
Scott
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Scott Goelzer
Physics Teacher
Coe - Brown Northwood Academy
Northwood NH 03261
603-942-5531 ext43
sgoelzer@coebrownacademy.com
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