Engines of Our Ingenuity

No. 1261:
EQUILIBRIUM

by John H. Lienhard

Click here for audio of Episode 1261.

Today, we try to keep our equilibrium. 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.

An acrobat is poised on a high wire. He walks a terribly unstable path. If his balance shifts, just a little, he'll fall away onto the hard dirt below.

Staying up there means staying in equilibrium. It means constantly shifting his weight so his tendency to fall to the left doesn't overcome his tendency to fall to the right. Once he loses his balance -- once his equilibrium is gone -- he's no longer unstable. He's merely falling through the air.

The words equilibrium and stability are poorly understood, because a person or a thing cannot be unstable if it isn't first in equilibrium. The two words are different faces of the same thing.

For example, did you know you can heat water far beyond its boiling point without its actually boiling? As you do, the water stays in equilibrium, but it grows increasingly unstable. To keep the overheated water from boiling you must protect it from any disturbance. When you bring the temperature far enough beyond the boiling point, the tiny motions of the water's own molecules become enough to knock it off its equilibrium. When that happens, the water boils so violently that it explodes -- often doing terrible damage. It's the pure equivalent of our falling tight-rope walker.

So equilibrium can take different forms. A marble caught in the bottom of a vee-shaped groove is in completely stable equilibrium. Jiggle it, and it only falls back to the bottom. Stable equilibrium is very a uninteresting rut.

We humans live in varying states of unstable equilibrium. When we walk, we achieve a brief unstable balance on one foot. Then we begin a non-equilibrium free-fall and catch ourselves on the other foot. When we call another person unstable, that doesn't say much. We're all unstable -- all the time. I suppose what we really mean is that person enters free fall without knowing how to grasp the next equilibrium state.

And when we say a person has a healthy equilibrium, we certainly aren't talking about someone in an uninteresting rut. More likely we're talking about a person who can walk a tightrope -- who can fall and recover. A healthy equilibrium is one that rides instabilities -- the little girl on roller skates, the man in a hang glider, the person who can lose one job and find another.

As words spill over from our technical vocabularies into popular speech, they lose important subtleties. We can make better use of the metaphors they express if we go back and remind ourselves what they first meant. When we do that, equilibrium loses some of its attractiveness. And instability becomes a thing we would not want to live without.

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

(Theme music)


Shamsundar, N., and Lienhard, J.H., Equations of State and Spinodal Lines -- A Review. Nuclear Engineering and Design, Vol. 141, 1993, pp. 269-287.

J.H. Lienhard and J.M. Stephenson, "Temperature and Scale Effects upon Cavitation and Flashing in Free and Submerged Jets," Journal of Basic Engr. , Vol. 88, No. 2, 1966, p. 525.



Photo by Lienhard and Stephenson (above) of a 3/32 in. dia. superheated water jet leaving an orifice at 110.5 ft/s. The temperature of the water is 298 F with a local boiling point of 206 F. The water is in a state of extremely unstable equilibrium.


Drawing by Maria Zsygmond-Baca, courtesy of Peter Gordon

Fanciful Image of Balance


The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.
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