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# [Phys-L] real world physics, maximum dissipation, high-altitude skydiving, etc.

• From: John Denker <jsd@av8n.com>
• Date: Thu, 18 Oct 2012 10:42:24 -0700

Here are a couple of examples of real-world physics.

Once upon a time I was in the seminar room at Cornell. The speaker
was some famous big-shot who was applying for a faculty position. He
had just finished a talk that touched on Prigogine's work on dissipative
structures, and on General Systems Theory including the idea of maximum
entropy production.

In the front row of the audience was Rolf Landauer, who had come over
from IBM Watson Lab for the occasion. Evidently Landauer and the
speaker knew each other.

After the talk, Landauer asked whether the maximum dissipation "theorem"
applied to electric circuits. He drew the following example circuit:

switch resistor
/
______/ ____/\/\/\____
| |
| |
| |
--- L
------- L inductor
--- L
------- |
| |
|_______________________|

Landauer: So the idea is that when we throw the switch, the current
in builds up more and more so that the dissipation in the resistor
approaches a maximum.

Speaker: Yes.

Landauer: And this is a general principle ...

Speaker: yes

Landauer: ... not a fluke ...

Speaker: yes

Landauer: ... so it applies to other similar circuits ...

Speaker: yes

Landauer: ... just as the first law and the second law of thermodynamics
apply to all circuits.

Speaker yes. Congratulations, thank God, you've finally figured it out.
It's just that simple.

switch resistor
/
______/ ____/\/\/\____
| |
| |
--- |
------- ___|___
--- _______ capacitor
------- |
| |
|_______________________|

Landauer: In this circuit, the dissipation does not build up to a maximum.
Quite the opposite. This disproves your central idea.
It's just that simple.

The room got very very quiet. I was just a puppy at the time, and had never
met Landauer. I'd never even heard of him ... but as you can imagine, I was
very impressed. Apparently he was the only person in the room with a correctly-
functioning BS detector. I immediately put some effort into finding out who he
was.
http://en.wikipedia.org/wiki/Rolf_Landauer

=====================

I mention this because on 10/14/2012 10:42 AM, Anthony Lapinski wrote:
record skydive today

Apparently the guy had problems because he got into a spin. Reportedly
the team was expecting something of the sort, because they had encountered
similar problems before.

It is well known that an aircraft can get into a spin, and that some
spin modes are very stable ... making them difficult /or impossible/ to
get out of. Spins feature prominently in the fatal accident statistics.
http://www.av8n.com/how/htm/spins.html

This connects to the first part of this note as follows: A spin mode can
be very stable, but it is neither the maximum-dissipation configuration
nor the minimum-dissipation configuration. Insofar as the guy was trying
for a speed record, he presumably would have preferred a configuration
with a higher rate of entropy production ... not to mention the obvious
safety considerations. In other words, the spin was potentially what
NASA calls "Criticality 1" i.e. an issue that could lead to loss of
mission, loss of vehicle, and loss of crew. All in all, I am astonished
that they did not design anti-spin features into the guy's jump suit.

Spins are insanely complicated, but some of the features are understandable
in terms of basic physics. For example, if a satellite is spinning with
constant angular momentum, basic notions of thermodynamic stability suggest
that it will be most stable spinning around the axis of maximum moment of
inertia. (For aircraft, this usually corresponds to a "flat spin" mode.)

=====================

This is also connected to the recent discussion of
Show the checks.

It is often hard to motivate the students to solve each homework exercise
once, let alone solve it twice (so that the second solution serves as a
check on the first).

Part of the motivation problem can be seen in the proverb:
If it's not worth doing, it's not worth doing right.

Consider the famous "monkey shooting" exercise. Students consider this
hokey and unrealistic. They do not spend very much of their lives hunting
falling monkeys using low-velocity bullets. You and I know that the
exercise illustrates an important fundamental principle, but the students
don't see it that way.

The cure for this IMHO is to discuss real-world applications of physics.
Life-and-death is a pretty good motivation. The high-altitude skydive
team risked the mission and risked the guy's life because they didn't
understand the physics of spins.

The devil is in the details and I regrettably do not have any lesson plan
for connecting the skydive/spin issue to the introductory physics class
in a way that allows students to understand the details, but I would at
least mention the part of the story that is understandable: rotational
kinetic energy, different moment of inertia about different axes, et cetera.
I would also take the opportunity to dispel the nonsense they may have
learned in biology class about maximum dissipation "principles". I would
then use it as an advertisement: If you continue in physics you will get
to the point where you understand the whole story.
-- People who are good at selling caffeinated beverages have a role
to play in society.
-- People who are good at physics also have a role to play in society.