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From: Peter Moylan <peter@pmoylan.org>
Newsgroups: alt.usage.english,sci.physics
Subject: Re: The antics of thermodynamics, the depravity of relativity, the
 bunkum of quantum
Date: Thu, 13 Mar 2025 11:32:07 +1100
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On 13/03/25 03:21, jerryfriedman wrote:
> On Mon, 10 Mar 2025 21:55:47 +0000, Bertitaylor wrote:

>>>> You must have been a lousy student in school, Moylan.

I don't normally blow my own trumpet, but it seems to be appropriate in
this case. In my high school matriculation exams I got first class
honours in Physics. First class honours again in first year university
physics. This was at a university (Melbourne) that is regularly ranked
number 1 in the country and among the top 50 in the world. And they
didn't run any Mickey Mouse "physics for beginners" subject. It was the
same Physics I as taken by the physics majors.

I haven't looked up where you got your degree, Bertie. If it was a
reputable university, you're doing your level best to damage its reputation.

> You will not find a textbook that says pressure is a vector or
> writes pressure using its notation for vectors or refers to the
> components of a pressure vector.  You probably will find or have
> found one that says in words "pressure is force divided by area"

Usually in a high school text, at a level where the pupils don't yet
understand vectors.

> or something similar, not realizing that some students could
> misinterpret that as saying pressure is a vector, but it's pretty
> certain that the book will get the equations right.
>
>> When we say tire pressure is so much, that means there is so much
>> force acting normally on the tire wall per square inch.
>
> That's true.  The magnitude of the force is given by the pressure
> and the area, both scalars, and the direction of the force vector is
> normal to the tire wall.
>
>> So it is a vector all right even though it may sound like a
>> scalar. Like velocity is a vector but speed is a scalar even though
>> they are dimensionally same.
>
> No, it's a scalar even though it may sound like a vector.
>
>>> You may have to read carefully and even pay attention to
>>> typefaces.  The textbooks I have use italic for vectors and
>>> boldface for scalars.
>>
>> Irrelevant and dishonest.
>
> Relevant--that's how books distinguish between vectors and
> scalars--and true.

There is generally a preface or section near the beginning of the book
that explains the notation.

In physics, more than in any other subject, textbook authors work hard
to ensure that their readers understand which quantities are vectors.

There are some, not very common, situations where a differential area is
treated as a  vector, and if so the differential force is the product of
a scalar (pressure) and a vector (area). (If you try to do it as the
product of two vectors, you get the wrong answer.) In those cases the
authors will ensure that you know what they're doing, and why.

-- 
Peter Moylan       peter@pmoylan.org    http://www.pmoylan.org
Newcastle, NSW