How to Torture a Wheel

A Redesign of the Exploratorium’s Wheel Dynamics Exhibit

ED333BWinter 2001Assignment 2

Prof. Shelly Goldman

March 10, 2001

James Sulzenjsulzen@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

A spinning wheeland its parts

axle

hub

rim

wheel spin

1

quick yank downwards of axlehandle

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

pinch

2

quick yank downwards of

axle handle

wheel spin

pinch

pinch

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.

3

enlargement

enla

rgem

ent

The wheel and hub actually have a very tight fit. Therefore the pinching is actually almost microscopic with no perceptible movement of the hub or axle. Contact between the hub and the axle occurs most tightly just slightly before the axle centerline. This results in a net pushing force to the left on one end of the axle and to the right on the other end.

hub spin

maximum point of contact

axle

edge of the hubaxle center-

line

net force

42) With

clockwise wheel spin

5The net effect is that there is a push at one end of the axle that pushes in one direction and a push at the other end

of the axle that pushes in the opposite direction. These

two pushes make the whole wheel try to rotate around

it’s center at right angles to the plane of the wheel spin.

3) Causes clockwise wheel rotation

1) A downwards yank on the axle

The Wonder of Wheels - What’s Happening

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

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

The Precession Pendulum

How Well Do You Know Your Bicycle Wheel?This is a multiple choice quiz displayed on a large screen. Visitors should use the free-standing 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?

sensor heads which measure precession

rate, precession angle, pitch angle,

and pitch change rate

slidable and removable

counterbalancing weights

spindle which allows wheel armature to freely move in horizontal and

vertical directions sensor which measures wheel

rotation rate

sensor which measures wheel

spin rate

collar and tightening nut which allow wheel axle to

be angled to horizontal

wheel spin

second wheel which can be attached to armature to counterbalance and counter-spin the other wheel

base containing instrumentation

electronics

Display and keyboard. This provides

measurement readouts and guides visitors

through various games and experiments to do

with the wheel apparatus

wiring connecting display with instrumentation

Wheel and Gyroscope Lab