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Re: Why would the center of the earth have zero gravity

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On 12/7/25 11:35 AM, Thomas 'PointedEars' Lahn wrote:
> Thomas 'PointedEars' Lahn wrote:
>> First of all, it is not good to think of gravitation as a property of an
>> object as in "Earth *has* gravity".  That is NOT how it works.  [This is
>> frequently taught wrong in schools.]  Instead, it is an *interaction*
>> _between_ objects.  According to Newton's theory, objects _attract each
>> other_ because they have non-zero mass.  So (according to Newton) it is not
>> so that "earth does gravity" but that Terra (_Earth_) has non-zero mass,
> 
> ["non-zero X" simply means "X is not equal to zero"]
> 
>> and so do other objects (including people like you and me), and so everything
>> is attracted to everything else.  (As the story goes, he realized that an apple
>> and Earth attract each other in the same as Earth and the Moon attract each
>                                ^^^^^^^^^^^
> in the same _way_
> 
>> other, and Earth and the Sun are attracted to each other: Gravitation was
>> *universal*, not limited to Earth.  Thus he could also explain how Kepler's
>> planetary orbits arose, and predict them.)
>>
>> One way to understand how the magnitude of the gravitational acceleration at
>> the center of Terra is approximately zero is to consider that a test object
>> with a negligible non-zero mass (a "test mass") located there will be
>> attracted gravitationally by all the matter that surrounds it (which also
>> has non-zero mass) in all directions of space approximately in the same way
>> (*exactly* in the same way if the planet were spherically-symmetric and had
>> a uniform mass density; we know that this is not so for any planet, but it
>> is still a good approximation), so the net gravitational force on it and its
>> net gravitational acceleration is approximately (would be *exactly*) zero.

I understand this in theory.  My problem is that if an apple falls from
a tree and hits you in the head, it hurts, despite that it falls from
the tree because of its gravetational attraction to the Earth, not to you.

So, if you drive in the ocean, as you go deeper the preasure
increases... not because of the gravity between you and water, but
because of the force of gravity between the water and the earth.  You
will still get the bends and be crushed by the pressure.  I am pretty
sure a similar result would happen in the center of a perfectly
shperical planet.  I suppose I am having trouble conceptually with the
idea that in the center of the planet you would have effectly null force
of gravity, but there is still a huge amount of pressure from all
directions   If I was in the center of the earth, my effective weight
might be zero, but the whole weight of the plaent still surrounds me.


> 
> Granted, the "incredibly unbroken" one-paragraph sentence above is far too
> long, and reminds me of a certain Dr. Fassbinder }:-)
> 
> An attempt to rewrite it:
> 
> One way to understand how the magnitude of the gravitational acceleration
> at the center of Terra is approximately zero is to consider a test object
> with a negligible non-zero mass (a "test mass") there.  It will be attracted
> gravitationally by all the matter that surrounds it (which also has non-zero
> mass) in all directions of space approximately in the same way.  So the net
> gravitational force on it and its net gravitational acceleration are
> approximately zero.
> 
check

> [The test object would be attracted *exactly* in the same way if the planet
> were spherically-symmetric and had a uniform mass density; we know that this
> is not so for any planet, but it is still a good approximation.]
> 

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Why would the center of the earth have zero gravity Popping Mad <rainbow@colition.gov> - 2025-12-05 03:25 -0500
  Re: Why would the center of the earth have zero gravity Jim Pennino <jimp@gonzo.specsol.net> - 2025-12-05 07:19 -0800
    Re: Why would the center of the earth have zero gravity Ruben Safir <mrbrklyn@panix.com> - 2025-12-14 01:10 +0000
      Re: Why would the center of the earth have zero gravity Jim Pennino <jimp@gonzo.specsol.net> - 2025-12-13 20:48 -0800
        Re: Why would the center of the earth have zero gravity Popping Mad <rainbow@colition.gov> - 2025-12-16 01:36 -0500
    Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-14 16:14 +0100
    Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-14 16:17 +0100
  Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-06 21:06 +0100
    Re: Why would the center of the earth have zero gravity Popping Mad <rainbow@colition.gov> - 2025-12-07 01:01 -0500
      Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-07 10:27 +0100
        Re: Why would the center of the earth have zero gravity Popping Mad <rainbow@colition.gov> - 2025-12-07 21:10 -0500
    Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-07 17:35 +0100
      Re: Why would the center of the earth have zero gravity Popping Mad <rainbow@colition.gov> - 2025-12-07 21:28 -0500
        Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-08 06:51 +0100
          Re: Why would the center of the earth have zero gravity Popping Mad <rainbow@colition.gov> - 2025-12-08 05:04 -0500
            Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-08 18:27 +0100
            Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-08 18:29 +0100
          Re: Why would the center of the earth have zero gravity Ruben Safir <mrbrklyn@panix.com> - 2025-12-14 01:03 +0000
            Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-14 03:42 +0100
    Re: Why would the center of the earth have zero gravity Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2025-12-07 19:15 +0100

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