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

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Thomas 'PointedEars' Lahn <PointedEars@web.de> wrote:
> Popping Mad wrote:
> ^^^^^^^^^^^
> Please modify your newsreader's settings so that you post under your real
> name.  It is considered polite here.  (As you can see, you can *also*
> include your nickname if you want.)
> 
>> 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.)
>>>>

You are rambling and you are off topic and not being helpful.


>>>> [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.]
>> 
>> 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.
> 
> That is not quite correct.  There is also a small gravitational force
> between the person and the apple.  

That is irrelevant to the point.  I think you are writing to hear
yourself.

I didn't ask or address a question about degrees of freedom of all the 
graviational forces that one can measure in this situation.  I was just
trying to understand why the pressure at the center, or as one
approaches the center of a planet increases to incredable levels which
the actual pravatational force once experiences reaches zero.  

You are not helping at all in addressing this.  You are all over the
place.  And your language is difficult to parse.  And now you have
detoured to be the first person is 40 years to ask me to change my 
news reader's settings.

I'd make an arm chair diagnosis, but it is not worth it.  No help is
coming from this post on usenet.  I'll need to find a different venue to
see if I can clarify this a little.



> And the apple attracts Terra (_Earth_) in
> the same way that Terra attracts the apple.  However, as the person is at
> rest relative to Terra, they consider the apple to be the moving object
> (this was one of Newton's great insights, building on Galilei's principle of
> relativity).
> 
> However, the strength of the force between the person and the apple is
> negligibly small compared to that between Earth and the apple because the
> mass of Terra is so much larger than that of the person:
> 
> The strength of the gravitational force between the person (M =~ 70 kg) and
> the apple (m =~ 10 g) at a distance of 1 m is approximately
> 
>   F = G M m/r^2
>     =~ 6.674 ? 10^-11 m^3/(kg s^2) ? 70 kg ? 0.01 kg/(1 m)^2
>     =~ 4.67 ? 10^-11 N.
> 
> The gravitational force between Terra (M =~ 5.97 ? 10^24 kg, R =~ 6371 km)
> and the apple 1 m away from the former's surface is approximately
> 
>   F = G M m/r^2
>     =~ 6.674 ? 10^-11 m^3/(kg s^2) ? 5.97 ? 10^24 kg ? 0.01 kg/
>        (6371 km + 1 m)^2
>     =~ 0.0982 N,
> 
> i.e. 9 orders of magnitude larger.  [This is based on Earth modeled as a
> point mass in its center.  One can see that if one divides that by the mass
> of the apple, one obtains approximately the average surface gravitational
> acceleration which therefore is (without air resistance) the same for all
> objects near the terrestrial surface (Galilei's discovery): ca. 9.8 m/s^2.
> <https://www.youtube.com/watch?v=oYEgdZ3iEKA>]
> 
> Whatever the total force on the apple while it is in free fall, when the
> apple hits the person, the person's atoms also exert an oppositely directed
> force on the apple, and vice-versa, that prevents the apple from continuing
> to fall freely towards the center of gravitation of Terra.  When the apple
> hits the head of the person, due to the sudden increase in pressure on the
> latter's skin/body and potential damage to the body from that, the nervous
> system of the person sends electric signals to the person's brain which
> interprets this as pain :'-)
> 
> [If there is no person and no other object, the atoms of the surface of the
> planet, exerting the same opposite contact forces, prevent the apple from
> continuing to fall freely.  It turns out that is how the apple has a weight
> (not: mass which it has regardless of gravitation) in the first place.]
> 
>> So, if you drive in the ocean, as you go deeper the preasure
>> increases... not because of the gravity between you and water, but
> 
> _gravitation_
> 
>> because of the force of gravity between the water and the earth.
> 
> Both; but the gravitational force between the water and the person is,
> again, negligibly small compared to the gravitational force between the
> water and the rest of the planet.  It is, however, the latter force that
> the person experiences *partially* as it acts on the surface of the
> person's body.
> 
>> You will still get the bends and be crushed by the pressure.
> 
> Yes, if you go to deep, because of what I explain above.
> 
>> I am pretty sure a similar result would happen in the center of a perfectly
>> shperical planet.
> 
> It would.
> 
>> 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,
> 
> _gravitation_
> 
>> but there is still a huge amount of pressure from all directions
> We are only talking about the *gravitational* force on the object, that
> would make *it* move, and *its* acceleration due to gravitation.  Consider
> a person to be in a person-sized hole filled with air, and the surrounding
> rock to be prevented by other forces from falling in.  That person would
> _float_ due to the mass of the rock that it is *surrounded* by.
> 
>> If I was in the center of the earth, my effective weight might be zero,
> 
> It would be, approximately.
> 
>> but the whole weight of the plaent still surrounds me.
> 
> That is *how* the above can be explained.

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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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