# The Naked Scientists Forum

### Author Topic: Another question on gravity hollow earth  (Read 3483 times)

#### Alan McDougall

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##### Another question on gravity hollow earth
« on: 15/07/2008 01:15:11 »
Please don't jump on me. I just want to use the highly unlikely hollow earth as a question.

If Earth was hypothetically hollow with a shell/crust of say a thousand kilometers.

Could one walk sort of upside down on the crust and if they could what would the gravity be. Or...............?? you clever people take over

The picture is just to help with question. Sort of walkng upside down on the inner crust if you get my drift .Would gravity pull us upward towards the surface or what guys.
« Last Edit: 15/07/2008 01:17:53 by Alan McDougall »

#### graham.d

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##### Another question on gravity hollow earth
« Reply #1 on: 15/07/2008 10:22:46 »
As was explained on the other thread, the gravity field from a spherical shell, on the inside of the shell is zero. The only way you could walk on the "inside" would be if it was spinning with sufficient speed. Try referencing the old SF book by Larry Niven called "Ringworld" where he had a similar concept but on a larger scale (since proved to be unstable as a matter of interest). The sun would have to be a lot smaller too.

#### lyner

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##### Another question on gravity hollow earth
« Reply #2 on: 15/07/2008 10:27:46 »
graham.d got there first but this is my effort:
For a non-rotating, uniform, hollow Earth NO - because of the arguments in the other thread - there is no net  gravitational force.
If you were not at the poles and the Earth were rotating then, once you got in contact and started to move in a circle, the  'ground' would be  pushing you towards the centre / axis, giving you the illusion of a gravitational force. With a 24hour day this force would be tiny! You could  jump very high.
For a fairly smooth inner surface, you would find yourself on the equator, eventually.,
« Last Edit: 15/07/2008 10:42:35 by sophiecentaur »

#### Alan McDougall

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##### Another question on gravity hollow earth
« Reply #3 on: 15/07/2008 14:11:03 »
graham,

Quote
As was explained on the other thread, the gravity field from a spherical shell, on the inside of the shell is zero. The only way you could walk on the "inside" would be if it was spinning with sufficient speed. Try referencing the old SF book by Larry Niven called "Ringworld" where he had a similar concept but on a larger scale (since proved to be unstable as a matter of interest). The sun would have to be a lot smaller too

Centrifugal force ,what it it was not spinning? Would we just flout in the empty space

By the way I am also a devote life long reader of science fiction and have read many of Larry Niven's stories.

Yesterdays nonsensical science fiction becomes todays science fact. Re: Arthur Clark's communication satelites etc etc
« Last Edit: 15/07/2008 14:47:41 by Alan McDougall »

#### graham.d

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##### Another question on gravity hollow earth
« Reply #4 on: 15/07/2008 15:44:48 »
If it were not spinning we would fall towards the central sun, assuming it has mass. Without any central sun, we would float about in zero-g.

#### Alan McDougall

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##### Another question on gravity hollow earth
« Reply #5 on: 16/07/2008 00:27:23 »
Graham,

Quote
If it were not spinning we would fall towards the central sun, assuming it has mass. Without any central sun, we would float about in zero-g

Thank you Graham that answers my question in full!!

Regards

Alan

#### lyner

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##### Another question on gravity hollow earth
« Reply #6 on: 16/07/2008 23:21:18 »
A McD
Quote
Yesterdays nonsensical science fiction becomes todays science fact. Re: Arthur Clark's communication satelites etc etc
But Arthur's ideas like that were never considered 'nonsensical' because they were based on Physics. The only unthinkable aspect about them was the cost and the uncertainty about suitable rocket power being available.
You have to draw the distinction between Scientific Consistency and total Fantasy.
Now, Space 1999 was clearly total bollocks from beginning to end - and will always be considered to be.
If you never saw the series just let it lie - let it lie!
« Last Edit: 16/07/2008 23:24:39 by sophiecentaur »

#### Alan McDougall

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##### Another question on gravity hollow earth
« Reply #7 on: 17/07/2008 01:26:24 »
sopthiec,

I agree that Arthur Clark orbiting satelites were firmly based on physics, but at the time they were considered far out possibilities. Grouping his advanced ideas with a nonsense in the same sentence was an unfortunate mistake and I am sorry

Regards

Alan

#### The Naked Scientists Forum

##### Another question on gravity hollow earth
« Reply #7 on: 17/07/2008 01:26:24 »