Today, let's think about rotation. The University
of Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
created them.
For years, Jearl Walker was
one of my science heroes. He wrote the column
called The Amateur Scientist in
Scientific American. Every month he
described another experiment we could run or
apparatus we could build. I was enormously proud
the two times he used experiments I'd submitted to
him.
Now I've found Walker's book, Roundabout.
It's a set of articles about rotation, and it opens
up an amazing world. He writes about tops,
boomerangs, and frisbees. He explains how the
dimples on golf balls reduce their air drag and how
a top can stand at a gravity-defying angle. He
shows how the complexity of billiards changes from
two dimensions to three when you play racquetball
-- how both games depend on players putting spin on
a ball.
Others have done those subjects, and Walker isn't
one to kick a dead horse. He adds to the story.
Instead of treating the old problem of pitching a
curve ball, he breaks new ground. He talks about
ballet, karate, amusement park rides, and spinning
coins.
Rotational motion is fraught with subtle traps.
Bodies rotate because they possess angular momentum
-- inertial motion that isn't easily stopped. The
fun begins when you see how momentum is conserved.
A spinning racquetball carries two kinds of
inertia, forward motion and rotation. When it hits
a wall, those inertias re-distribute, and odd
things happen. The ball may nearly stop short while
it picks up a lot of spin. We've all watched
spinning coins convert their motion from a clean
straight-up rotation to increasingly wobbly motion
in the tortuous process of falling down.
Walker shows how ballet movements create optical
illusions. In a well-executed grand
jeté the dancer does a running jump.
Then she seems to hover in midleap. The trick is to
lift her arms as she rises. That lowers her center
of gravity at the top. As she comes down she lowers
her arms to compensate for her falling center of
gravity. So she appears to move in a straight line
(like an airplane) instead of in a parabola (like a
baseball.) Walker deconstructs ballet into a dozen
such strategies for managing momentum.
The game changes in an amusement park ride. We are
now part of the motion instead of outside
observers. Now we experience what the coin or the
racquetball does. Those rides shift our reality by
expanding upon the one simple force of gravity
we're used to. They rob us of our normal up and
down. If we turn our head during rotation, we bring
virtual forces to bear on our inner ear, and they
overturn a lifetime's logic of forces and
reactions.
In 1687 Newton boiled all this down into three
simple laws of motion. Jearl Walker's wonderful
book gives an inkling of the universe contained in
that core of simplicity. He reminds us just why
elementary physics is such a wondrous, never-ending
source of uncanny behavior -- just why the world is
as interesting as it is.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
