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*From*: "Robert Cohen" <Robert.Cohen@po-box.esu.edu>*Date*: Mon, 23 Jun 2008 21:12:04 -0400

Sorry about that - I misinterpreted what you meant when you said "if you already know the weight of the air column, you don't need...to assume hydrostatic balance."

I thought you meant that the pressure equals the weight per area whether there is hydrostatic balance or not. I figured hydostatic balance was the same as "mechanical equilibrium", which you indicated was a necessary assumption.

[I think we agree that the integration is only necessary to show that the P=wt/A can be derived from hydrostatic balance or mechanical equilibrium or whatever we want to call it]

________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of John Denker

Sent: Mon 6/23/2008 8:35 PM

To: Forum for Physics Educators

Subject: Re: [Phys-l] Atmospheric pressure calculus deduction

On 06/23/2008 03:42 PM, Robert Cohen wrote:

Can you expand on why one needn't assume hydrostatic balance? I can

easily imagine an atmosphere that is not in hydrostatic balance

(e.g., constant pressure with height) in which the surface pressure

would not equal the weight per area.

Well, I can imagine such things too, and that could be a problem,

but in such cases integrating g rho dz doesn't solve the problem.

In particular, suppose their is a huge wind blowing over an

airfoil-shaped hill. In that case it's not even entirely obvious

what you mean by pressure --(static pressure? dynamic pressure?)--

and even once you decide that, knowing the weight of the air

column and/or g rho dz isn't going to tell you the pressure.

In any case, the point remains that the integral of g rho dz *is*

the weight of the air column, so if you knew the weight at the

beginning you wouldn't need to do the integral. IMHO that's

a pretty robust result. (This assumes g is constant, which is

good to very high accuracy in the troposphere and stratosphere.)

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**References**:**[Phys-l] Atmospheric pressure calculus deduction***From:*"CARABAJAL PEREZ, MARCIAL ROBERTO" <MCARABAJALP@repsolypf.com>

**Re: [Phys-l] Atmospheric pressure calculus deduction***From:*"Pete Lohstreter" <plohstreter@MAIL.HOCKADAY.ORG>

**Re: [Phys-l] Atmospheric pressure calculus deduction***From:*Brian Whatcott <betwys1@sbcglobal.net>

**Re: [Phys-l] Atmospheric pressure calculus deduction***From:*"Pete Lohstreter" <plohstreter@MAIL.HOCKADAY.ORG>

**Re: [Phys-l] Atmospheric pressure calculus deduction***From:*Brian Whatcott <betwys1@sbcglobal.net>

**Re: [Phys-l] Atmospheric pressure calculus deduction***From:*"Robert Cohen" <Robert.Cohen@po-box.esu.edu>

**Re: [Phys-l] Atmospheric pressure calculus deduction***From:*John Denker <jsd@av8n.com>

**Re: [Phys-l] Atmospheric pressure calculus deduction***From:*"Robert Cohen" <Robert.Cohen@po-box.esu.edu>

**Re: [Phys-l] Atmospheric pressure calculus deduction***From:*John Denker <jsd@av8n.com>

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