Bobbing Heads

[from Gary Cziko 930323.1700 GMT]

Bill Powers (930319.0730) said:

The next time you're in an elevator (alone), just before the
doors close, hold your hand outstretched and watch it. When the
acceleration begins, you'll see your hand sightly dip (rise) for
a moment and come back to the former position. You will also hear
the machinery start, feel increased (decreased) pressure on the
soles of your feet, and feel increased (decreased) effort in
muscles all over your body, particularly those holding the arm
out. Use the parenthesed expressions if the elevator starts
downward. You will see the hand doing approximately what the
above diagram shows, although without the sharp peak (which will
be upside down if the elevator descends).

It's quite a coincidence that Bill should bring this up just as I was
observing the same phenomenon--but travelling sideways, not up and down.

You don't need an elevator and don't need to travel alone. Just watch
people's head bob back and forth in a car (or bus or train or whatever) in
"reponse" to changes in acceleration. As driver when I slow down to make a
stop I see my passengers' heads move forward then back again to where it
should be. As I accelerate from the stop, the heads move backwards before
coming to rest again a la verticale. No problem with explaining this using

But what is particularly intriguing is that the driver's head does not seem
to move nearly as much. When I'm driving, I control the acceleration of
the car and so I seem to use this advance knowledge of impending
accelerations to minimize my head bobbing. This phenomenon is what I'm
having some trouble understanding as in PCT terms since it appears to be a
good example of what a "normal" psychologist would probably refer to as

I can nicely demonstrate the difference in head bobbing between driver and
passenger by becoming a passenger myself as I drive. I can do this by
using my cruise control in third gear (manual tranmission) set at about 30
miles per hour. At this low speed and gear ration my cruise control is not
very stable and keeps swinging above and below the target speeds--and my
head bobs back and forth like everyone else's in the car.

(Bill, I suspect that my cruise control does this because in third gear
there is not enough slowing for the loop gain of the system?).--Gary


Gary Cziko Telephone: 217-333-8527
Educational Psychology FAX: 217-244-7620
University of Illinois E-mail:
1310 S. Sixth Street Radio: N9MJZ
210 Education Building
Champaign, Illinois 61820-6990