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From: jerry.friedman99@gmail.com (jerryfriedman)
Newsgroups: alt.usage.english,sci.physics
Subject: Re: The antics of thermodynamics, the depravity of relativity, the bunkum
 of quantum
Date: Mon, 10 Mar 2025 19:54:55 +0000
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On Sun, 9 Mar 2025 9:50:06 +0000, Peter Moylan wrote:

> On 09/03/25 09:58, Phil wrote:
>> On 08/03/2025 22:46, Bertietaylor wrote:
>
>>>>> It's just as implausible as the suggestion (easily disproved)
>>>>> that the pressure is zero at the centre of the earth.
>>>>
>>>> The pressure is most certainly zero at the centre of the stars
>>>> and planets. Read a first year book on physics.
>>>
>>> Which will say that within an enclosed surface with mass the net
>>> gravitational force or pressure is zero.
>
> Read that first year book yourself. Did you find the words "or
> pressure"? No, I didn't think so. You've tried to conclude something
> about the pressure from the gravitational force. That doesn't work,
> because they are different quantities.
>
> Gravitational force, like all forces, is a vector quantity. It has a
> magnitude and a direction. That makes it possible that a number of
> nonzero vectors can sum to zero; and, indeed, that is what happens
> inside a spherical shell.
>
> Pressure is a scalar. If you add two pressures, you get a higher
> pressure. There's no such thing as a negative pressure to cancel out the
> first pressure.
>
> Think of a cone, or similar shape, whose point is at the centre of the
> earth. You can separate out a section with thickness dr, and write down
> the force balance equation for that slab. (This, too, is first year
> physics.) From that you get a differential equation for the pressure as
> a function of radius. No matter what simplifications you make, you will
> get the same conclusion: the deeper you go, the higher the pressure.
> Which is something that ocean divers can confirm from their own
> experience.

Even swimming pool divers.

> At the centre of the earth, the gravitational force is zero but the
> pressure is at a maximum.
>
>> Presumably, by an analogous argument, the pressure at the centre of a
>>  balloon is also zero?
>
> Actually, the gravitational force at the centre of a balloon is zero, if
> you count only the force due to the balloon itself. But of course, you
> do have to count the attraction from the earth as well.
>
> Either way, what you conclude about the gravitational force says nothing
> about the pressure. They're different quantities.

They're different, but they are connected.  Since
the gravitational force at the center of the Earth is
0, you can conclude that the pressure /gradient/
there is 0.  Hint to Arindam.

--
Jerry Friedman

--