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No. 332:
A Mechanics Class

Today, we watch some imaginative teachers at work. 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.

The other day I passed a colleague's office and found him stretching a length of key chain out on his desk. He let part of it hang over the edge and then released it. The chain gathered speed and slid off onto the floor. He timed its fall and then hurried to class. He asked his students to predict how long the chain took to slide off. The students were fascinated because they were called upon to predict reality -- a fresh reality -- a reality that not even their teacher had yet mastered. It had the imperfections of reality. How important was friction? Did the chain slide straight, or was there a wavy movement?

The next day I passed again. He was writing the experiment up for a journal called The Physics Teacher. He'd caught the students' interest, and he was going to share it. I looked at his copy of the journal to see what else was in it.

One article was by a teacher who watched MacGyver on TV. In case you don't know, the MacGyver character used an encyclopedic knowledge of science to get out of scrapes in prime time. In one episode, MacGyver had been poisoned, and the metal container of antidote had slipped out of reach down a storm sewer. He saved himself by beating a metal rod to induce magnetism in it. Then he lowered the rod on a string to get the antidote. The author talked about the process and, finally, gave the script his blessing.

I turn the page. Here's another University of Houston professor with a board across his head. An assistant is hammering a nail into it. The man's head is hardly bumped. The students are asked to explain why.

This inventory of examples is a celebration of the classroom at its best. I also have tricks like these up my sleeve. I like to show how a rubber band shrinks when you heat it and then ask my thermodynamics students to make sense of that unlikely behavior.

This sort of thing might look frivolous, but it lies at the heart of understanding science. Between knowing the formalisms of science and really feeling them in your bones lies a huge chasm. Many students never learn to cross that chasm, and a few teachers don't care if they do.

Science is turned from alien mystery into a living companion only when formal rules take an active role in explaining nature. That's why Richard Feynman's famous experiment with the Challenger O-ring was so powerful. When he dipped a pliant little rubber ring in ice-water and it turned rigid, brittle, and useless before our eyes, we suddenly knew in a deeply personal way why astronauts had died. True learning takes place when a student becomes a participant in using science to master a mysterious commonplace world.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Faucher, G., Ferromagnetism and the Secret Agent. The Physics Teacher, January, 1988, pp. 30-31.

The instructor with the sliding chain experiment is Professor James Casey, Mechanical Engineering Department, University of Houston. He was teaching the junior level course, MECE 3336, Mechanics II. (Since this episode was aired, Casey moved to The University of California at Berkeley.)

Casey's equation for the chain's movement is:

x\b = c\b + (1 - c\b) cosh (nt)


t = the time after sliding begins

x = the length of the chain hanging over the end at time t

b = the length of the chain hanging over the end when t = 0

c = the maximum length of chain that can hang over the end without starting the chain into motion.

c = fL/(1 + f)

f = the moving coefficient of friction

L = the overall length of the chain

n = [(1 + f)g\L]½

g = the acceleration of gravity

The instructor with the nail-in-the-board experiment was Professor Tom Hudson, University of Houston physics professor and a gifted user of demonstration experiments. His article on the experiment is in the journal cited above.