🚀 go-pugleaf

In the recent thread on tippy-tops we mentioned footballs, class rings,
etc. as objects which have the 'tipping axis' behavior. How did we miss
the obvious one: a hard-boiled egg spun on its side will rise up and spin
on an end. I was reminded of this while reading for pleasure Martin
Gardner's marvelous collection: The Encyclopedia of Impromptu Magic
(Magic Inc., Chicago) This is available from magic shops (about $40 now,
I think) and has a many of the classic ideas for physics demos. Some of
these are now appearing Monthly in Martin's contributions to the Physics
Teacher journal.

An obvious point about these. Since the center of mass of these objects
RISES, this means that some kinetic energy is converted to potential
energy, and this is done through the action of force of friction. If one
asks, "What is the force acting on the body which raises its potential
energy?", the answer is: friction.

If one asks, what is the force acting on an automobile which keeps it
going, and even can accelerate it, the answer is: friction. The forward
force of friction of the road acting on the tires. Remove the friction, and
you'd get nowhere. Remove the friction at the table surface and your
tippy-top won't tip.

For some reason, one of the many *false* concepts students acquire is the
notion that friction can only *remove* energy from a body. This represents
a serious misunderstanding of F = ma, perhaps a failure to realize that F
in F = ma is the NET EXTERNAL force on the body of mass m. Internal
forces don't belong in this equation, they cancel from Newton's third law.

        -- Donald