Today, we look at the question of size. 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.
We understand things on the
scale that we experience them. We're often suprised
by how differently things behave when they're very
small or large. To see what I mean, try this
experiment. First find a very small and a very
large metal sphere -- say a BB and the big steel
ball used in the shot put. Now drop each of them
from a height of a few feet into a swimming pool.
You'll see that the shot
splash isn't at all like a scaled-up BB splash.
The large shot sends out a sheet of water that
breaks into a fine spray of drops. There are only a
few drops in the BB splash.
Before we had today's computer simulations, that was
how we could always tell whether a naval battle in a
movie was a scale model or full size. The
splashes never looked right in the scale model.
(Watch an old WW-II vintage movie about a naval
battle. You'll see.)
I once knew a badly crippled construction worker.
He'd been working on the ledge of a building that
was being demolished when he saw a two-ton scoop
swinging toward him -- very gently -- very slowly.
He put out his hands to stop it as he might have
stopped a child on a swing. And when it reached
him, it very gently crushed him. His experience
with playground swings had grieviously misled him
about the behavior of two-ton scoops.
Engineers think a lot about making scale models of
big prototypes. We wouldn't get very far if we had
to make full-size wind-tunnel tests of a Concorde
SST. The trick is to set the conditions in a small
model so its behavior is similar to the large
prototype. For example, we really could use a BB
experiment to learn about the large shot hitting
water, if we changed two things. The BB would have
to move much faster than the shot, and we'd have to
put just the right amount of detergent in the water
to cut its surface tension.
What the theory of modeling does is to tell us how
to stretch the dimensions of all the variables --
masses, speeds, material properties -- into
universal values. When we do this, funny things
happen. For example, we can learn about the
movement of microorganisms in our body fluids by
making laboratory experiments with large objects
moving very slowly through cold honey.
The problem of modeling is one part of a general
problem we have to face whenever we design things.
We have to find ways to see what's not obvious to
our eyes. We have to find ways to predict
complicated behavior before it becomes part of our
experience.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
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