# The Naked Scientists Forum

### Author Topic: Is there zero gravity at the centre of the Earth?  (Read 17354 times)

#### Aaron Thomas

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##### Is there zero gravity at the centre of the Earth?
« on: 14/11/2010 13:30:03 »
Aaron Thomas asked the Naked Scientists:

Dear Chris,

Thanks for the great show, I am always listening to your pod-casts which I think are fantastic.

I have a question/s which I hope you can find the answer/s too........

Q. 1.0. Apparently at the earth's core there is an area of zero gravity, how big would this area be?

Q. 1.1 If this area exists then would the mass within this area of zero g be affected by the moon or the suns gravity?

Q 1.2 If the moon or the suns gravity causes this area of zero g to move then could this be how our magnetic field is generated? could this be the dynamo?

Sincerely

Aaron Thomas

What do you think?
« Last Edit: 14/11/2010 13:30:03 by _system »

#### Bill S

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##### Is there zero gravity at the centre of the Earth?
« Reply #1 on: 14/11/2010 17:14:06 »
Aaron, I don't have the scientific know-how to try to answer your question. No doubt Chris will, but just a thought in the meantime.
1.0 Think of a rotating disc, at the centre is a stationary point.  How big is it?  My thought is that it must be infinitesimally small.  Would the same not apply to the zero-gravity point at the centre of the Earth?
1.1 If this area has measurable mass, it must be accompanied by measurable gravity (?). If it appears to be gravitationally neutral, that must be because of some external influence (?). If it is infinitesimally small, how could we measure any effect it might have?
1.2 Gravity is a two-way process; if this is an area of zero gravity, would it not be un-influenced by external gravitational sources?

#### maffsolo

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##### Is there zero gravity at the centre of the Earth?
« Reply #2 on: 14/11/2010 19:27:14 »
Actually, I believe that there is no absolute center to the earths axes.
There is a wobble that makes the inner core slash around. The core is liquid produced by the gravitational pressure on the surface presses the earths mass.
This wobble also affects the magnetic poles  continuously being relocate.

This wobble is caused by the pulling affect of the associated orbit of the moon the earth around the sun and the earth's daily revolution.

This it the same as tethering a bucket of water with a rope and orbiting it in a big circle around your head. You can feel the forces produced at the grip of your hand.

http://alansthoughts.com/resources/2-Why+The+Earth+Wobbles+with+figures.doc

http://en.wikipedia.org/wiki/Polhode

#### JP

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##### Is there zero gravity at the centre of the Earth?
« Reply #3 on: 15/11/2010 04:20:22 »
Q. 1.0. Apparently at the earth's core there is an area of zero gravity, how big would this area be?

When people say that, they usually mean they're assuming the earth is a perfectly uniform sphere.  In that case, when you're down below the ground, the bits of earth below you are pulling you down, while the bits above you are pulling you up, and the bits to the side of you are pulling you sideways in various directions.  If you're at the point exactly in the center of the earth, the earth is pulling you equally in all directions at the same time, so that the forces all balance each other and you float there, apparently weightless.  Of course, there is still gravity from the earth, but it's just perfectly balanced!  This would only happen at a tiny point precisely at the earth's center.

Since the earth isn't perfectly spherical and uniform, this point doesn't really exist in practice.  Let's assume it does, or at least there's a small region

Quote
Q. 1.1 If this area exists then would the mass within this area of zero g be affected by the moon or the suns gravity?
Yes.  Only the earth's gravity perfectly cancels at that point.  Other sources of gravity outside the earth would effect things at that point.

Quote
Q 1.2 If the moon or the suns gravity causes this area of zero g to move then could this be how our magnetic field is generated? could this be the dynamo?

Someone who knows geophysics better can probably answer this question more fully.  I believe most of the magnetic field has to do with currents in the liquid outer core of the earth.  The outer core is liquid primarily because of the pressure of the earth's outer layers pressing down on it, the residual heat from when the earth formed, and radioactive minerals releasing heat.  The moon's gravity squeezes the earth a bit (which you can see quite readily since it's squeezing of the oceans causes tides), so that has some effect too.  I don't think it's the main effect, and it acts on the whole earth at once, not just on the very center.  The sun's gravity doesn't really squeeze the earth, so it wouldn't have too much of an effect.

#### Airthumbs

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##### Is there zero gravity at the centre of the Earth?
« Reply #4 on: 18/11/2010 01:50:43 »
Thanks everyone for the interesting answers to my questions,

So what I think your all saying is;

That there could be an area ,or something?, of neutral, or negative?, gravity in equilibrium with gravity at the centre of the Earth?, having no mass, infinitesimally small, existing in the gravitational central axis?, effected by external gravitational influences, its effects unknown?

#### SteveFish

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##### Is there zero gravity at the centre of the Earth?
« Reply #5 on: 18/11/2010 03:58:52 »
A quick addition. Although gravity would be neutralized in the center of the earth, the pressure from the whole mass would be enormous.

#### peppercorn

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##### Is there zero gravity at the centre of the Earth?
« Reply #6 on: 18/11/2010 10:10:36 »
A quick addition. Although gravity would be neutralized in the center of the earth, the pressure from the whole mass would be enormous.
The physics of how to overcome this is clearly explained in the Hollywood film 'The Core'!

#### imatfaal

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##### Is there zero gravity at the centre of the Earth?
« Reply #7 on: 18/11/2010 10:13:25 »
Aaron - basically, if you could hollow out the centre 10 metres of the earth (insert any number here), and ignoring the fact that you would burn up and that the pressure would be unbearable (and Steve, isn't The Core a terrible film?) , you could float around in there just like you see the astronauts on the international space station.  it would have zero gravitational attraction at all points.

Quote
having no mass, infinitesimally small, existing in the gravitational central axis?, effected by external gravitational influences, its effects unknown?
Rephrase it and we will try to answer

#### JP

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##### Is there zero gravity at the centre of the Earth?
« Reply #8 on: 18/11/2010 11:40:04 »
A quick addition. Although gravity would be neutralized in the center of the earth, the pressure from the whole mass would be enormous.
The physics of how to overcome this is clearly explained in the Hollywood film 'The Core'!

Well, yes.  With your unobtanium suit, you'd be just fine!

#### peppercorn

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##### Is there zero gravity at the centre of the Earth?
« Reply #9 on: 18/11/2010 12:15:46 »
A quick addition. Although gravity would be neutralized in the center of the earth, the pressure from the whole mass would be enormous.
The physics of how to overcome this is clearly explained in the Hollywood film 'The Core'!

Well, yes.  With your unobtanium suit, you'd be just fine!
Damn! It's at the dry cleaners!

#### Airthumbs

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##### Is there zero gravity at the centre of the Earth?
« Reply #10 on: 18/11/2010 13:33:27 »
I think what I am trying to get at is the size of the area of zero "g" and what effect it might have if any.  In previous comments someone suggested that this area would be very small.   Could it really be big enough to float in?  Also once I can find out how big this area is, then I will then be able to possibly visualise it, moving around within the core being effected by the Earths wobble and other gravitational influences, in relation to its position. If the earth is not perfectly spherical and thus causes the center of mass to change then would this point not be moving? I also think that an area which must be as as hot as the surface of the sun at the very center of the earth having no gravity is quite interesting. What also interests me now is how big this area would be in the Sun or even the biggest star VY Canis Majoris? Would this area interact with material in the inner core, meaning what does a substance composed primarily of a nickel-iron alloy at a pressure of 330 to 360 gigapascals and at 5,430 degree centigrade do when in zero "g" would its chemical properties change?

#### imatfaal

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##### Is there zero gravity at the centre of the Earth?
« Reply #11 on: 18/11/2010 16:35:32 »
Aaron - as most of the replies have stated, the shell theorem relates to a hollow sphere.  The gravity in a solid sphere is related by 1/r; that is to say that in a mineshaft reaching to the centre of the earth (ignoring impossible heat and pressure) a miner would feel less attraction to the centre of mass as he descended.

you seem to be conflating mathematical ideas with real world problems - we cannot make a hollow room at the centre of the earth, let alone the sun; but we can predict the gravitational attraction if we did.

Theoretically, no matter how big the celestial object and what percentage was hollowed out, as long as it is spherically symmetrical there is no net gravitational attraction within the shell.  Practically, on an engineering side there are real experts in these forums and geographically bass, jb and their ilk on the geology board seem to be massively knowledgeable.  But you have to ask the right question!

#### SteveFish

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##### Is there zero gravity at the centre of the Earth?
« Reply #12 on: 18/11/2010 16:45:13 »
Sheesh, I don't know what all the fuss is about. Just take your tinfoil suit, your shades, and a couple of beers and it could be great!

Aaron, the actual point of zero gravity would be infinitesimally small, but the region of microgravity (e.g. in close earth orbit) where you couldn't personally tell the difference, would be quite large, perhaps on the order of tens of miles. Steve

#### Bored chemist

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##### Is there zero gravity at the centre of the Earth?
« Reply #13 on: 18/11/2010 19:10:48 »
If you hollowed out nearly the whole of the earth and just left the outermost mile (or whatever)  as a thin spherical shell, the gravity anywhere inside it would be zero.

It wouldn't just be a point of zero g in the middle; the whole of the region would have zero g.

#### SteveFish

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##### Is there zero gravity at the centre of the Earth?
« Reply #14 on: 19/11/2010 02:23:11 »
Bored chemist, I don't understand what you say at all. Please explain. It seems to me that because of the inverse square law you would feel a much greater attraction of gravity from the side of the hollow sphere that is very close than from the other side that is 7,000 miles away. Steve

#### Geezer

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##### Is there zero gravity at the centre of the Earth?
« Reply #15 on: 19/11/2010 05:46:20 »
Bored chemist, I don't understand what you say at all. Please explain. It seems to me that because of the inverse square law you would feel a much greater attraction of gravity from the side of the hollow sphere that is very close than from the other side that is 7,000 miles away. Steve

Steve, you need to check out Shell Theorem. I keep getting it wrong myself

#### Bored chemist

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##### Is there zero gravity at the centre of the Earth?
« Reply #16 on: 19/11/2010 06:55:15 »
Bored chemist, I don't understand what you say at all. Please explain. It seems to me that because of the inverse square law you would feel a much greater attraction of gravity from the side of the hollow sphere that is very close than from the other side that is 7,000 miles away. Steve
If, for example, you are very near the bottom of the shell, it's true that the bottom bit is nearer, but most of the shell is above you. More stuff further away balances out less stuff nearer.
Or, as Geezer says, check out the theorem.
http://en.wikipedia.org/wiki/Shell_theorem#Inside_a_shell

#### SteveFish

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##### Is there zero gravity at the centre of the Earth?
« Reply #17 on: 19/11/2010 16:04:05 »
Thanks guys, you made my day.

#### Bill S

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##### Is there zero gravity at the centre of the Earth?
« Reply #18 on: 19/11/2010 23:44:56 »
Quote from: BC
If, for example, you are very near the bottom of the shell

Which bit is the bottom???

#### Bored chemist

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##### Is there zero gravity at the centre of the Earth?
« Reply #19 on: 20/11/2010 16:46:14 »
Quote from: BC
If, for example, you are very near the bottom of the shell

Which bit is the bottom???
The other end,from the top.
Of course, it doesn't really matter but it makes it easier to refer to the stuff above you and below you rather than trying to keep it abstract.

#### Airthumbs

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##### Is there zero gravity at the centre of the Earth?
« Reply #20 on: 26/11/2010 06:23:21 »
If the fact that we are in so called free-fall orbit around the sun would indicate that this area of zero gravity is not effected by the gravity of the moon or sun then;

Why do we have tides on the surface of the planet where gravity is at full strength?  Is this not effected by the pull of the moons gravity?  If the moon can exert such an influence on the surface of the Earth then would it also do something to the huge area within the Earth that is less effected by gravity?

Professor Brian Cox seems to think and has stated that gravity is responsible for the commencement of the reaction at the centre of stars.  Also this extract: "In the nebular hypothesis, the majority of the mass of the dust cloud collects at the center. The intense gravitational forces present ultimately lead to nuclear fusion taking place. As most of the matter initially present in the nebula is hydrogen, the process of hydrogen burning takes place." (University of Winnipeg)

How can the above be correct when at the centre of a large mass such as the Sun, nebulae, there is essentially zero gravity in total contradiction to this theory! So I would hypothesise that nuclear fusion must take place at the point where gravity is the strongest, on the surface?

Finally, maybe if you were able to find your way to the centre of a large interstellar object you would not float at all in zero gravity but your atoms would all get pulled apart by the forces pulling on you in every direction.  I imagine this would not be a very pleasant experience, but does it explain nuclear fusion?

#### Geezer

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##### Is there zero gravity at the centre of the Earth?
« Reply #21 on: 26/11/2010 07:57:03 »
How can the above be correct when at the centre of a large mass such as the Sun, nebulae, there is essentially zero gravity in total contradiction to this theory! So I would hypothesise that nuclear fusion must take place at the point where gravity is the strongest, on the surface?

Ah! You may be missing an important point here. Gravity at the center of a star is zero, but the pressure is enormous. The pressure is caused by gravity pulling all the material towards the center. The pulling force (gravity) reduces near the center, but the enormous mass of material above the center creates the pressure.

The pressure results in fusion, but gravity was still responsible for creating the pressure.

#### Airthumbs

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##### Is there zero gravity at the centre of the Earth?
« Reply #22 on: 26/11/2010 09:00:47 »
Thanks geezer, so it is the pressure combined with heat that causes fusion and this is a result of the forces of gravity bringing the mass together in the first place.  Ahhhh now it makes sense.

What about the tide thing and the Moon with relation to the zero gravity area? Any suggestions......

#### imatfaal

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##### Is there zero gravity at the centre of the Earth?
« Reply #23 on: 26/11/2010 11:34:28 »
Aaron - Newtons Shell theorem (the one that tells us, amongst other things, that gravity within the above mentioned shell is zero) does specify that the material of the shell is symmetrical and spherical for zero gravitational attraction to apply.  The presence of the moon and sun (and to a much much lesser extent) the other planets do disrupt this symmetry.  I believe, but I am not absolutely sure, that the effect of the moon could be worked out using simple F = G.m1.m2/r2 - and I dont think it would be very much at all.

#### Airthumbs

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##### Is there zero gravity at the centre of the Earth?
« Reply #24 on: 27/12/2010 14:44:35 »
I cannot help notice that everyone is using empty shells to describe the gravity on earth. Why is that please?
« Last Edit: 29/12/2010 05:02:41 by Aaron Thomas »

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##### Is there zero gravity at the centre of the Earth?
« Reply #24 on: 27/12/2010 14:44:35 »