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Re: Impedance &c.



Wolfgang Rueckner writes:

There is a chapter on this very question in a wonderful little book
entitled "Similarities in Physics" by John Shive and Robert Weber.
(That's the same Shive that gave us the Shive Wave Machine, which has
become a staple in lecture demonstrations.) In general, if you
regard impedance as the ratio of cause to effect [and let's NOT get
into cause/effect arguments here], then in electrical circuits the
ratio would be the applied voltage to resulting current and the power
is their product.

There is a small religious problem here. Part of the catechism we
physics teachers are raised with is the proposition that force does not
cause velocity. Any percipient bicycle rider knows that this is untrue,
but still we do try to get students to believe it. To introduce force
and velocity as a cause-effect pair would pose more than philosophical
problems; it would be a heresy. Of course they are a cause-effect pair
in steady state motion of, say, an object being propelled in a viscous
medium.

I want to relate the idea of impedance to other tasks: the throwing of
a ball, for example. Why is it easier (as Fred Bucheit points out) to
impart kinetic energy to a thrown baseball than to a thrown BB or a
bowling ball? How can this problem, matching human throwing ability to
either of the latter tasks, be approached using a transformer? The
problem was solved in prehistory, of course. We have the atlatl, the
archery bow, and the trebuchet as examples of impedance matching
devices of increasing sophistication. How do we describe the
improvements to the archery bow in terms of impedance matching? I want
a systematic treatment centered on a concept, impedance, that my
intuition tells me is there. The concept is well developed in
electricity and less well developed in mechanics.

Shive and Weber give examples from many areas, whether they be wave
physics or mechanics, etc. and talk about impedance matching devices
(transformers) in all of these situations for maximum transfer of
power. The book is a lot of fun.

Wolfgang has given me one valuable reference of the sort I seek,
"Similarities in Physics". Unfortunately our library does not shelve
this book, and Amazon is enigmatic (check it out). Blackwell's shows it
at ninety pounds (!) and out of print.

Leigh