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Re: [Phys-l] question about Bernoulli



I agree. The macroscopic approach has never been in dispute on the thread. The original question was asked on the microscopic level. Unfortunately, it seems to me that the explanations so far have at some point appealed to the macroscopic (like the pressure-density relation) and then switched to what the molecules must be doing for that to come about.

I don't think it is unfair to ask "why" a bullet leaves a gun and to offer an explanation that the gas molecules on one end of the bullet are moving faster and hitting the bullet often because of their high temperature and that the gas molecules of the atmosphere on the other end of the bullet are hitting it less often. "Why" is often just a shorthand for asking how a process develops. Every equilibrium flow started as a transient flow - asking how this came about is not inappropriate.

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Philip Keller [PKeller@holmdelschools.org]
Sent: Tuesday, November 23, 2010 8:59 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] question about Bernoulli

I understand the distinction between "why" and "how do we know". But there are many situations in physics where the "how we know" refers to something "global" and our intuition craves an explanation that is "local". For example, I see the requirements of continuity as global. But when someone then asks "why" do the molecules speed up, I think they are looking for something like JM's explanation.

This has come up on the list before in other settings. For example, conservation laws offer a "global" explanation of events, but many students (and teachers) prefer the "local" explanation where the individual objects obey Newton's laws. Like when the teacher on the turntable holds a spinning wheel above his head and then grabs the wheel, thus starting to spin. Explanation #1: the system had angular momentum before and it still has angular momentum after, so the teacher must spin. Explanation #2: the teacher starts to spin because of an external torque applied by the bicycle wheel -- the reaction to the torque the teacher applied to the wheel.

Like the conservation laws, Bernoulli's eqn seems to have the power to predict what will happen on the global scale while leaving the local scale mysteriious. I think that is the feeling that got this particular thread started.
________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] on behalf of John Denker [jsd@av8n.com]
Sent: Tuesday, November 23, 2010 8:09 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] question about Bernoulli

On 11/23/2010 04:58 PM, LaMontagne, Bob wrote:
iabatic, then pressure and density have to change in the same
direction - so it's kind of chicken and egg. On the microsopic level,
why does the density lower? Mass continuity requires rho*A*v to be
constant in equilibrium flow, so small A will require higher v - but
why a rho change? - it's not needed with a liquid obviously - so why
with gas molecules with lots of space between them?

If the gas molecules speed up, they must have suffered more
collisions from the direction of the wider pipe and fewer from the
direction of the narrower pipe - so we're back around the circle
again.

If you insist on asking "why" then the laws of physics are
always going to lead you around the chicken-and-egg circle.
The laws of physics need to say what happens. They sometimes
say how it happens. The fundamental laws rarely if ever say
why it happens. This is important. This is what sets physics
apart from metaphysics and philosophy. This has been called
Galileo's more important contribution, and has been called the
epoch, i.e. Day One of modern science.

The slightly different question "how do we know" will get a
lot more traction.

In the case of a fluid flowing from a big pipe into a little
pipe, we know it *must* speed up because we know about
continuity.

Note that this "cause" is the cause of the knowing, not the
cause of the speeding up. We can perfectly well infer the
speed from the diameter *or* infer the diameter from the
speed (in this context).
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Forum for Physics Educators
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l