Engines of Our Ingenuity
No. 1313:
ROTATION

by John H. Lienhard

Click here for audio of Episode 1313.

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)


Walker, J., Roundabout: The Physics of rotation in the Everyday World. New york: W. H. Freeman and Company, 1985.




Pictured above is an object called a rattleback. It is a solid boat-shaped object who's curved bottom is asymmetrical. When you spin it clockwise, it gradually converts its angular momentum to a rocking motion. It finally stops rotating, sits and rocks for a moment while it reconverts that momentum back to counterclockwise rotation. Then it commences to rotate in the direction opposite the way it started out, now in its maximum moment-of-inertia mode. It gives the illusion of being determined to spin only in the counterclockwise direction. (Walker discusses rattlebacks in Chapter 6 of his book.)


The Engines of Our Ingenuity is Copyright © 1988-1998 by John H. Lienhard.

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