Ballistics vs. Feedback Demo

[from Gary Cziko 930131.1705 GMT]

Bill Powers has made some illuminating arguments about how a feedback
system can be expected to be faster than an open-loop, ballistic system.
I've found what I think is a simple way to demonstrate this to which I
would like to get some reactions. You will need a large, smooth table, two
coins, and a stopwatch. An assistant to work the stopwatch may also be

Stand or sit beside the table. Then take two coins (B for balistic and F
for feedback) and put them on one side of the table with one directly above
the other. Now, slide one coin (B) from where it is to the other side of
the table ballistically. This means giving it a quick push with your
finger so that it slides shuffleboard-style across the table and comes to
rest when the laws of physics says it must. The time taken from the
initial push to the final stop should be timed. Now move the remaining
coin (F) from where it is (and from where B started) to the new location of
coin B and time how long it takes. Do this as quickly as you can by
keeping your finger on the top of the coin as you slide it across the
table. Compare the two times.

When I do this, I get times for coin F which are ALWAYS faster than coin B,
feedback taking only about 60% of the time of ballistics (for example, .86
seconds for coin B compared to .52 for coin F). It seems that the two
coins take off at about the same speed but that coin B starts to slow down
immediately after the push while coin F continues to accelerate until
getting very close the target and then a very sophisticated ABS braking
system takes over to decelerate the coin sharply right before the target.

Somehow I feel that the physics of coins sliding on tables doesn't make
this a watertight demo that closed loop can be faster than open (limbs
don't have much friction, do they?), but I'd like people to try the demo
and get some reactions from the more physically enlightened people on the

P.S. According to my notes, the equipment kit for the compleat low-tech
portable PCT demonstrator now includes rubberbands, pencils & paper (or
chalkboard & chalk), coins, tennisball, and a stopwatch.


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

[From Rick Marken (930131.1030)]

Gary Cziko (930131.1705 GMT)

I've found what I think is a simple way to demonstrate this to which I
would like to get some reactions.

This is absolutely your best demo yet!! I love it. I hope it holds
up to the scrutiny of the physically inclined types. But I think
there are all kinds of possible variations on it that can satisfy even
the most hard nosed members of the net. It's really ingenious, Gary;
nice going.