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A nice classroom demonstration of stability (and the close
relationship between static friction and rolling friction) is to take
two (large--6-8 inches long, so they can easily be seen) toy cars and
put them on an inclined plane. Tape the front axle on one and the
rear axle on the other, so that the taped wheels don't turn. Then get
the class to guess which car will roll straight down the ramp and
which won't. My experience has been that most students will guess
that the one with the taped rear wheels will be stable and the other
one will not be.
They are quite surprised when the opposite proves to be the case. The
explanation is that once the rear wheels are locked, they are
providing less of a retarding force than the front wheels, and so any
misalignment of the front and rear wheels will cause the car to start
spinning about a vertical axis unstably, so that the small
perturbation gets magnified. This illustrates why two features of
many modern cars, front-wheel drive and anti-lock brakes are
extremely helpful in keeping cars under control in reduced friction
situations.
Hugh
--
Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>