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On 05/26/2013 10:08 AM, Dr. Holly Priestley wrote:
One uses these on a piano stool, no?
Yes ... and no. There are a lot of demos you can do with a
bicycle wheel, some of which involve a stool ... and some of
which don't.
I reckon the folks who are recommending a lead-weighted wheel
are probably setting up for a demonstration of conservation of
angular momentum, using a stool or something similar.
In contrast: If you want to demonstrate gyroscopic precession
and gyroscopic stability, you don't need or want a stool ...
and you don't need or want extra weight. The main thing that
matters is the /ratio/ of angular momentum to weight, so a
rapidly-spinning wheel is just the ticket. You don't need much
mass, provided that whatever mass there is lies far from the
axis. A mountain-bike, road-bike, or racing-bike tire works
fine. Demos in this category include:
1) Start with the wheel spinning in a vertical plane. Plan a
way to re-orient it to a horizontal plane. Then grab the handles
and do the experiment. Did your plan work?
The tactile perception of the forces required is a big part of
the lesson. As Confucius may have said, one hands-on feeling
is worth a thousand words.
2) Start with the wheel spinning in a vertical plane. Support
its weight by one finger at the /end/ of one of the handles.
Observe that it precesses. Note the direction of the applied
torque, and the direction of precession. Explain the result
by adding bivectors edge-to-edge.
-- Do not say anything about cross products.
-- Do not say anything about any right-hand rules.
Some discussion, with diagrams, can be found here:
http://www.av8n.com/how/htm/motion.html#sec-gyro-rule
3) You can demonstrate the principle of operation of an
aircraft rate-of-turn gyro.
4) You can demonstrate the principle of operation of an
aircraft artificial horizon gyro.
5) You can demonstrate the principle of operation of an
aircraft directional gyro.
============================
On 05/26/2013 09:28 AM, Dan Beeker wrote:
Consider your audience. For those with short arms I find the
smaller 20" wheels work much better. Avoids chaffing of the chin
with the tire.
I'm all in favor of considering the audience ... but unless you're
giving physics lessons to the Lollipop Guild, there won't be many
students who can't handle a lightweight 700mm tire.
Of course /if/ you make it heavier, /then/ you might have to
make it smaller, because nobody wants to hold something heavy
at arm's length.
======================================================
Note that the same tire can be used for demos having nothing to do
with angular momentum. For starters:
a) Here is a non-equilibrium situation.
b) Here is an equilibrium situation with negative stability
c) Here is an equilibrium situation with neutral stability
d) Here is an equilibrium situation with positive stability
Here's the picture:
http://www.av8n.com/how/htm/equilib.html#fig-eq-stab
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