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#42659 - 02/27/12 05:43 AM Is pressure a scalar or a vector?
gan Offline
Member

Registered: 07/20/10
Posts: 55
P=F/A whereas F is a vector... Pressure is the force act PERPENDICULARLY to a surface area. That's why I believe pressure is a vector quantity. And what I have read in Stress Tensor, Pressure should be a vector...

Most book say that P=lFl/A why lFl ???

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Of Interest?
#42660 - 02/27/12 09:10 AM Re: Is pressure a scalar or a vector? [Re: gan]
TheFallibleFiend Offline
Megastar

Registered: 06/08/05
Posts: 1940
Loc: http://thefalliblefiend.blogsp...
Good question. I never thought of it before. Wiki has an answer at http://en.wikipedia.org/wiki/Pressure

"Pressure is a scalar quantity. It relates the vector surface element (a vector normal to the surface) with the normal force acting on it. The pressure is the scalar proportionality constant that relates the two normal vectors:"

That is, both F and A are vectors - collinear and facing opposite directions. I don't remember learning this in any of my classes - but it's been 30 years since I took mechanics.

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#42696 - 03/02/12 10:04 AM Re: Is pressure a scalar or a vector? [Re: gan]
Orac Offline
Megastar

Registered: 05/20/11
Posts: 2080
Loc: Currently Illinois, USA
Originally Posted By: gan
P=F/A whereas F is a vector... Pressure is the force act PERPENDICULARLY to a surface area. That's why I believe pressure is a vector quantity. And what I have read in Stress Tensor, Pressure should be a vector...

Most book say that P=lFl/A why lFl ???


So many things to clear up ... lets start here.

=>Pressure is the force act PERPENDICULARLY to a surface area

Think about a tire with air in it. Are all the molecules inside hitting the surface perpendicular? .... probably not hey the point Amit is making above.

So pressure is the CUMLATIVE PERPENDICULAR result of a series of smaller forces which are probably not perpendicular.

The result is perpendicular because that is the way we choose to measure it is probably not the direction of alot of the actual collissions.

Can you have a negative force? Answer no.

You either apply a force or you don't that is because we have Newtons first law ... remember a body with no force stays at rest. Hence no negative forces are possible ... what would a negative force even do ... make an unmoving body unmove more ????

So pressure can only be positive and it actually acts in all directions ... it's our containment into a vessel or virtual vessel (think magnetic confinement etc) and application of a surface area that creates a directionality but you still don't get positive and negative pressure.

What we are saying is pressure is scalar it's our measurement technique that makes it vector in nature.


Edited by Orac (03/02/12 10:08 AM)
_________________________
QM crazy, always wrong, never believe me, sad and broken and now lost credibility and I cry wolf.

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#42702 - 03/02/12 04:14 PM Re: Is pressure a scalar or a vector? [Re: gan]
Bill S. Offline
Megastar

Registered: 08/20/10
Posts: 2917
Loc: Essex, UK
Originally Posted By: O
Can you have a negative force? Answer no.


I am very relieved to read this. I have spent some time and effort unsuccessfully trying to allign some kind of negative force with the concepts of negative energy and negative mass.

Thanks Orac, I can stop waisting my time on that! smile
_________________________
There never was nothing.

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#42710 - 03/04/12 07:09 AM Re: Is pressure a scalar or a vector? [Re: gan]
Bill S. Offline
Megastar

Registered: 08/20/10
Posts: 2917
Loc: Essex, UK
Isnít it embarrassing when you look at one of your past posts, and the first thing that you see is an error; like putting an extra ďlĒ in align? You canít even blame the computer for that!
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There never was nothing.

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#42712 - 03/04/12 09:10 AM Re: Is pressure a scalar or a vector? [Re: Bill S.]
Bill Offline
Megastar

Registered: 12/30/10
Posts: 1367
Loc: Oklahoma, USA
Originally Posted By: Bill S.
Isnít it embarrassing when you look at one of your past posts, and the first thing that you see is an error; like putting an extra ďlĒ in align? You canít even blame the computer for that!

Sure you can blame your computer for that. It is always a computer error when one of my posts gets goophed up. After all I couldn't make an error like that.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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#42714 - 03/04/12 10:13 AM Re: Is pressure a scalar or a vector? [Re: gan]
Bill S. Offline
Megastar

Registered: 08/20/10
Posts: 2917
Loc: Essex, UK
Quote:
Sure you can blame your computer for that.


Thanks, Bill. I shall be ETERNALLY grateful. smile
_________________________
There never was nothing.

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#42715 - 03/04/12 10:20 AM Re: Is pressure a scalar or a vector? [Re: gan]
Bill S. Offline
Megastar

Registered: 08/20/10
Posts: 2917
Loc: Essex, UK
Amit, yours is not a stock avarar; so you should be able to answer the question that has been floating around for a while. How do you insert a custom avatar?
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There never was nothing.

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