How to Torture a Wheel
A Redesign of the Exploratorium’s
Wheel Dynamics Exhibit
ED333B
Winter 2001
Assignment 2
Prof. Shelly Goldman
March 10, 2001
James Sulzen
jsulzen@stanford.edu
Introduction
•
•
•
•
I am just finishing a study with Dan Schwartz of, among other things, how people learn about
the dynamics of bicycle wheels. I learned much about wheel dynamics during the course of
the study, and I welcome the opportunity to apply this knowledge to extending a related
exhibit at the Exploratorium in San Francisco.
The Exploratorium’s Wheel Dynamics exhibit has a number of hands-on activities allowing
visitors to explore the effects of spinning wheels and gyroscopes (see Photo 1). It has the
following exhibit elements:
– Two free-standing wheels that visitors can manipulate.
– A chair to sit in and use a wheel to spin oneself around via precessional effects (see
Photo 2).
– A two-sided gyroscope mounted on a swivel base (see Photo 3).
– A hook and chain assembly to support a bicycle wheel for creating gyroscopic effects
(see Photo 4).
– Signs describing these activities and some of the science behind them.
Based upon my observations, I perceive several problems with the exhibit:
– Visitors hardly ever learn much of anything from their experience with the exhibit.
– The exhibit is not terribly engaging - visitors have a difficult time discerning what the
meaningful activities are without reading the various signage. As a result almost no
visitors stay more than a minute or so before moving on.
– Even if a visitor makes the effort to stay and to engage, they still are unable to learn
anything with the present exhibit. Instead, they get to witness various phenomena, but
are forced to walk away with no increased understanding of deeper affects or cause of the
phenomena.
– The exhibit does support collaborative exploration very well.
The net result is that the exhibit seems to produce at best a “gee whiz” experience.
Why this Problem is Important
Wheels in general, and bicycle wheels in particular, are quite common ordinary everyday
experiences. The Physics of wheel dynamics is taught in most all High School and college
freshman Physics courses. Yet despite the ordinariness of it, and even after twice explicitly
studying the subject in a scholastic career, virtually no student (nor even instructors as it
happens) has much if any intuitive understanding of how all the forces operate in wheel
dynamics. You can show the phenomenon from Photo 4 (the precessing wheel) to almost
anybody and no one is able to offer a satisfactory let alone intuitive explanation of why the
wheel does not pivot down when it is spinning.
To quote a seventeen-year veteran of High School Physics teaching, “I have to tell you that
my understanding of the Physics is not supported by my intuitive sense… Even after I’ve
used Physics to explain why the wheel stays up [when suspended from one side by a rope] I
still want to ask ‘why doesn’t it fall down?’ It’s magic.”
All wheel dynamics courses are structured around the Principle of Conservation of Angular
Momentum. There are a number of fairly difficult to comprehend equations and concepts
that are required to master this approach; these provide no basis for an intuitive
understanding of just what it is that happens with spinning wheels when you try to
manipulate them. The Exploratotium exhibit’s explanation (see Photo 5) is no better (and in
fact, I think is perhaps misleading). As one woman visitor said to me, after having read it,
“Why’d they even bother putting it there? It doesn’t tell me anything.” This seems to be
pretty much the same case with how Physics teaches the subject to students: They
memorize the formulas for the test but inevitably lose the learning because no physical
intuition is provided about gyroscopic phenomena.
As such, there seems to be a distinct need for illustrating ways to develop physical intuitions
about wheel dynamics.
Observations
•
•
•
•
•
On March 9, I spent the last hour of the day at the Exploratorium’s wheel exhibit and made
the following observations (recorded over a 45 minute period). (See Photo 1.)
Twenty-seven people in some thirteen groups stopped by and either looked at or interacted
with the exhibit in some way. About half the groups had two people interact and all the others
only had one person. Someone from four of these groups was observed reading signs (see
Photo 6). None of the readers or participants seemed particularly enlightened (no expressions
of comprehension or amused delight, etc.), nor were they able to articulately respond about
what they had learned when I queried some of them directly as they were leaving the exhibit
area.
With one exception, the main form of interaction with the exhibit was to pick up one of the
two free-standing bicycle wheels and spin it. Seven of the individuals sat in the chair and
tried to turn themselves with the wheel (four of whom had difficulties and three of whom
were fairly successful - see Photo 2). The only non-wheel spinning participant was a
gentleman approximately in his sixties who saw me set up and use the hanging bicycle wheel
to to try out the precession part of the exhibit (Photo 4); he brought back his wife several
minutes later to show her.
Not a single person so much as examined the gyroscope.
In contrast, in the same time period, 34 people jumped on an adjacent exhibit that allowed a
single participant to spin in place (see Photo 7). Four people were observed to read its sign.
This activity level markedly contrasts with the relatively lukewarm bicycle wheel
participation by the 13 people mentioned above.
What Makes this Difficult to Learn
•
•
•
•
•
•
One of the purposes of the study I did with Dan was to try to identify what made it so difficult to
learn about how a bicycle wheel worked. My tentative conclusions are as follows:
Visual/Kinesthetic Confounding: First and foremost, the visual and kinesthetic systems confound
each other when playing with the wheel. The intuitive visual sense is that spinning objects
should behave more or less similarly to non-spinning ones. However, the kinesthetic experience
with the wheel is communicating something quite different from what the visual system seems to
expect. People seem to have a strong bias for preferring visual inputs in these situations with the
net result that most people become rather confused and deadlocked in developing learning
around the wheel.
Transitoriness: The physical effects are very dynamic and transitory in nature. This makes it very
hard to make meaningful observations as effects whip right past observers before they can quite
perceive what is happening, let alone what caused it to happen.
Measurement: It is particularly difficult, when manual manipulation is the only available
investigatory technique, to meaningfully calibrate or measure physical effects. Even
ameliorating the visual and kinesthetic confounding still leaves it very difficult to feel at a
sensory level, let alone a perceptual one, just what effect produces exactly which result and to
what degree.
Mechanism: The mechanism of force transfer from the spinning hub to the axle is actually rather
hidden, subtle, and very non-intuitive for most people.
Multiple Simultaneous Manipulations and Effects: In manipulating the wheel, people often tend
to do several things at once without realizing it (such as change the spin direction, spin speed,
and axle torque rate). This confounds the observations in terms of isolating which input(s)
caused the apparent result.
The net result is that people just don’t do very well in learning about the wheel when on their
own.
Design Solution
This seems like a very ripe opportunity for exactly an Exploratorium hands-on approach
to help visitors develop some sort of physical intuition about wheel dynamics. If we can
go even further and actually teach people how and why wheels behave so unexpectedly,
then visitors can leave with some real world knowledge that connects with every day
experience.
The following pages illustrate several new exhibits which are designed to follow the
Exploratorium’s hands-on style of learning and to allow visitors to gain some intuitive
and intellectual comprehension of wheel dynamics. They are ordered from least
expensive and easiest to implement to most expensive and involved.
Proposed new exhibits:
1) New explanation of wheel dynamics
2) The Precession Pendulum - Wheel hung as a pendulum
3) The Interactive Wheel Quiz
4) Wheel and Gyroscope Laboratory
The Wonder of Wheels - What’s Happening
1
2
A spinning wheel
and its parts
rim
Yanking the axle down on one side of
the wheel while it is spinning causes
the axle to pinch tightly at opposite
sides of the hub.
wheel
spin
pinch
wheel spin
hub
wheel spin
axle
pinch
pinch
pinch
quick yank
downwards of
axle
handle
The net effect is that there is
a push at one end of the axle 5
3) Causes clockwise
that pushes in one direction
wheel rotation
and a push at the other end
of the axle that pushes in the
opposite direction. These 2) With
two pushes make the whole clockwise
wheel try to rotate around wheel spin
it’s center at right angles to
1) A downwards
the plane of the wheel spin.
yank on the axle
3
This diagram is a
greatly exaggerated
enlargement
illustrating how the
axle pinches extra
tightly at the ends of
the hub when it is
yanked down. The
pinching causes
some of the wheel’s
spinning motion to
be transferred to
each end of the axle.
quick yank
downwards of
axle handle
The wheel and hub
actually have a very tight
edge of fit. Therefore the pinching
axle centerthe hub is actually almost
microscopic with no
line
axle
perceptible movement of
hub the hub or axle. Contact
spin between the hub and the
4
axle occurs most tightly
just slightly before the axle
net
centerline. This results in
force maximum
a net pushing force to the
left on one end of the axle
point of
and to the right on the
contact
other end.
The Precession Pendulum
collar and
tightening nut which
allow wheel axle to
be angled to
horizontal
lines drawn on
floor underneath
apparatus to help
visitors discern
the curve motion
of the wheel as it
precesses
rigid rod and rigid
connection so that the
entire wheel apparatus
is suspended from the
ceiling and acts as a
giant pendulum
focused light source
shining as a spot on the
floor so that it is easy to
see exactly what path the
wheel traces out as it
swings
How Well Do You Know Your Bicycle Wheel?
This is a multiple choice quiz displayed on a large screen. Visitors should use the freestanding wheels to see if they can figure out which are the correct answers. Assume that
the wheel is held in one hand and is spinning in a clockwise direction when answering
the questions.
1. What will happen if you hold a wheel axle with one hand, spin the wheel clockwise, and you
twist upwards with the wrist that is holding the axle?
2. What will happen if you move the wheel's axle parallel to itself (i.e., left/right or up/down
without twisting the axle)?
3. What will happen if you just hold the wheel's axle at a 45-degree angle to the horizontal?
4. What will happen if you attach a rope to one side of the wheel to hold it up?
5. What will happen if you suspend the wheel by using a rope that attaches to both sides of the
wheel;
6. (Follow-up to previous question) What happens if the wheel’s axle is not quite parallel to the
horizontal?
7. What happens if you twist your wrist (and the axle) upwards while spinning the wheel faster
versus slower?
8. (Follow-up to previous question) Or if you spin it clockwise versus counter-clockwise?
9. Draw a line that represents the motion described by the end of the axle if you gently sweep
your hand horizontally back and forth, bending only at the wrist?
10. What happens if you quickly twist the axle, say by flexing your wrist upwards, versus doing
so more slowly versus doing it very slowly?
second wheel which can be
attached to armature to
counterbalance and
counter-spin the other
wheel
Display and keyboard.
Wheel and Gyroscope Lab
This provides
measurement readouts
spindle which allows
and guides visitors
wheel armature to freely
through various games
move in horizontal and
and experiments to do
sensor which
vertical directions
measures wheel with the wheel apparatus
rotation rate
collar and
tightening nut
which allow wheel
axle to be angled to
horizontal
slidable and
removable
counterbalancin
g weights
sensor heads which
measure precession
rate, precession
angle, pitch angle,
and pitch change rate
wheel
spin
base containing
instrumentation
electronics
sensor which
measures wheel
spin rate
wiring connecting display
with instrumentation