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
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
Shamsundar, N., and Lienhard, J.H., Equations of
State and Spinodal Lines -- A Review. Nuclear
Engineering and Design, Vol. 141, 1993, pp.
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.
The Engines of Our Ingenuity is
Copyright © 1988-1997 by John H.
Drawing by Maria Zsygmond-Baca,
courtesy of Peter Gordon
Fanciful Image of Balance
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