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Re: [Phys-l] question about Bernoulli

All analogies fail at some point. If they didn't they wouldn't be analogies. So I guess that's a reason not to use analogies. After all, those "naive people milling about" might just go away thinking that molecules have feet and can move about at will, despite the fact that they are instructed during the simulation that molecules don't have a choice what to do once they are moving in a given direction. The other molecules and the walls of the container dictate what they do. I use teeter totters as an analogy for how variables in an equation are related, but teeter totters have a fulcrum, and certainly people will go away thinking that there's a fulcrum in an equals sign. You can use a line of people moving toward and away from a given point, sending people out to walk faster at regular intervals and then determining the frequency at which these people arrive at the target point as an analogy for the Doppler effect, but of course that means those people will think that sound and light waves are composed of individual objects with feet.

Searching for analogies and simulations that will help people better understand a concept is one part of physics education. As I said, the important thing is that the analogies and simulations do not introduce (important, not trivial) misconceptions. I don't understand why searching for a model for the Bernoulli effect is a bad thing. If it's not possible, it's not possible. But someone claiming authoritatively that it is not possible does not make it so. Difficult to accomplish is not the same as impossible. Many claim it's impossible to help people get a conceptual understanding of some of the basics of quantum mechanics without any math, but it is in fact possible to do so. It simply takes creativity. And being creative in helping people understand physics seems a worthy goal of physics educators.


Bill

On Nov 26, 2010, at 7:24 AM, John Denker wrote:

On 11/25/2010 09:07 PM, William Robertson wrote:
Is it possible to understand the
Bernoulli effect in terms of molecular motion and changes in molecular
motion on a level that makes the effect "obvious" once one pictures
what the molecules are doing? Until I get through everything in this
thread, I don't yet know the answer to that question.

Here it is, a week and a half later, and I still don't understand
why that is a question. I hate to belabor the obvious, but the
obvious answer remains the correct answer:

The fluid slows down when it climbs a pressure gradient.

-- That is true heuristically and formally.
-- That is true microscopically and macroscopically.
-- That is true locally and everywhere.
-- That is true in every other way.

Also: If you apply Bernoulli's principle outside its domain of
validity, you will get wrong answers.

Absolutely. I do a simulation with teachers in which
they pretend to be gas molecules with others being their container.

Maybe that's the problem. That's not a particularly apt model.
The velocity of the people is determined almost entirely by what
they choose to do with their feet. Maybe in some fantasy-world
they would choose to accurately model the pressure and velocity
of a fluid flowing through an orifice ... but in the real world
they won't. They don't know how.

And as Bob L. observed, even if you have a computer simulation
where the density, velocity, and pressure are known to be accurate,
it is not trivial to instrument the simulation so as to make it
easy to comprehend the patterns and laws involved. It can be done,
but it involves a level of sophistication far removed from a bunch
of naive people milling around.
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