# The Naked Scientists Forum

### Author Topic: What would happen at the centre of a tunnel through the Earth?  (Read 6669 times)

#### thedoc

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##### What would happen at the centre of a tunnel through the Earth?
« on: 12/03/2012 08:50:03 »

If you could dig a hole straight through the Earth and decided to jump into the hole, what would happen when you reached the centre point?

What do you think?
« Last Edit: 12/03/2012 08:50:03 by _system »

#### imatfaal

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #1 on: 12/03/2012 10:54:25 »
1.  You would burn up - it's quite hot down there.  But, if you were to ignore heat...
2.  You would hit the side of the hole.  At the equator you are doing at 1600kmph laterally, you would end up going faster than the hole... But if you were to dig pole to pole...
3.  Those two provisos aside; when you reached the centre you would be travelling very quickly - enough to get you all the way to the other side of the earth.  As you passed the centre you would start slowing down again, and (barring air resistance) you would slow down to a stop just as you appeared on the other side of the world.
4.  You would then oscillate back and forward - in an ideal setup with no losses due to friction - forever; each time reaching the surface, stopping and then falling back down to the other side of the world.

BTW Newton calculated how long it would take!  And even more amazing is that the theoretical time (ignore bumps and friction) is exactly the same for all tunnels through the earth - even those that doe not go through the centre.  The shallow tunnels are shorter, but you get a slower top speed.

There is a wikipage here

#### syhprum

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #2 on: 12/03/2012 14:40:30 »
A there and back trip would take the same time as a satellite orbiting at ground level, if you held your arms out to create some friction you would stop at the centre and just float.
The transit time depends only on the density of the planet not its size.
« Last Edit: 12/03/2012 14:43:55 by syhprum »

#### Sprool

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #3 on: 13/03/2012 11:26:43 »
that would be one heck of a fairground ride.

#### RD

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #4 on: 13/03/2012 11:34:19 »
There is a wikipage here

#### yor_on

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #5 on: 14/03/2012 04:33:48 »
A jojo, with a slight twist methinks :)
You have those 'imaginary forces' to consider too, as the Coriolis force

#### CZARCAR

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #6 on: 15/03/2012 14:07:14 »
A jojo, with a slight twist methinks :)
You have those 'imaginary forces' to consider too, as the Coriolis force
a string moreso than a spring?

#### CZARCAR

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #7 on: 15/03/2012 14:08:30 »
alphbetically, the p comes be4 the t

#### wucko

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #8 on: 17/03/2012 00:04:44 »

If you could dig a hole straight through the Earth and decided to jump into the hole, what would happen when you reached the centre point?

What do you think?

hi, im glas i found this question, years ago , arround 2004 i think i asked myself this exact question. Adding to it:

what is gravity? at the centre of the earth (im in a heat proof tunnel :)) does the earth's mass: shrink me or stretch me.

what i learned from the question is: that there is actually no exact understanding og what gravity actually is in phisics, so the question remains valid and i think unanswered.

#### wucko

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #9 on: 17/03/2012 00:24:10 »

If you could dig a hole straight through the Earth and decided to jump into the hole, what would happen when you reached the centre point?

What do you think?

hi, im glas i found this question, years ago , arround 2004 i think i asked myself this exact question. Adding to it:

what is gravity? at the centre of the earth (im in a heat proof tunnel :)) does the earth's mass: shrink me or stretch me.

what i learned from the question is: that there is actually no exact understanding og what gravity actually is in phisics, so the question remains valid and i think unanswered.

in the gravitational centre of the sphere i think you woul suddenly stop (forever) since the force that was pulling you lets say from south pole towards north pole, in the exact gravitational centre of the sphere would suddenly transform into forces that either pull you appart from centere in all directions equally strong (hense imemediate stop, full stop, forever) << OR ? >> posh towards the centre itself (shrink you) from all directions.

i think there is no answer to A OR B? since gravity isnt defined at all. and this question is the important one:

if now we imagine that we could freeze time  while at the exact ventre of the sphere: i bet 50% of you would vote: the sphere's mass is pulling you apart, and 50% would vote its pushing you in (shrinking you)

pull or shrink?
;)
what IS gravity?

#### Soul Surfer

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #10 on: 17/03/2012 11:42:54 »
There is no net force from the body and no stress on the body if it is at the centre of gravity of that body.  It is easily provable (by integrating the equations) that the gravitational acceleration (force) inside a massive uniform thick spherical shell is zero at all points inside the shell.  Outside of the shell it behaves as if all the mass in the shell was concentrated at the centre of the sphere and when you move through the shell from the outside to the inside the gravitational acceleration (force) falls linearly from the value outside of the shell to zero linearly.

There may be slight deviations from this for non uniform, not precisely spherical and rotating bodies but there is always a centre of gravity where the forces are zero.
« Last Edit: 17/03/2012 11:45:38 by Soul Surfer »

#### Soul Surfer

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #11 on: 17/03/2012 11:51:49 »
As an aside there was a science fiction story of a planet with a thin crust a large liquid core containing two heavy bodies orbiting each other inside the core.  The local gravitational field was then continually varying in magnitude and direction with interesting effects.  Notably it was possible to move around in a wheeled vehicle on the reasonably flat surface of the planet by waiting until it was downhill in the direction you wanted to go letting off the brakes and coasting but putting the brakes back on again and staying where you are when it was uphill in the direction you wanted to travel!

#### wucko

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #12 on: 18/03/2012 20:57:28 »
There is no net force from the body and no stress on the body if it is at the centre of gravity of that body.  It is easily provable (by integrating the equations) that the gravitational acceleration (force) inside a massive uniform thick spherical shell is zero at all points inside the shell.  Outside of the shell it behaves as if all the mass in the shell was concentrated at the centre of the sphere and when you move through the shell from the outside to the inside the gravitational acceleration (force) falls linearly from the value outside of the shell to zero linearly.

There may be slight deviations from this for non uniform, not precisely spherical and rotating bodies but there is always a centre of gravity where the forces are zero.

thank you,

this would mean i would be "stretched" in all cases - or not? I gather that AT the centre the G=0, on my trip from North to South id be pulled towards the centre with F(m to the South) and "away" from the centre with F(m to the North), this F becomming smaller as i further travel to the centre and finnaly 0  in the centre.

id assume, that if the G force didnt drop while traveling closer to the centre, but accumulate (or even stay the same), the body would collapse into itself, such as in a black hole. in such a case only, i would be "squeezed" in?

Best
W

#### Soul Surfer

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #13 on: 18/03/2012 23:30:38 »
Inside the cavity there are no forces on you due to the gravitating sphere.  You would not be stretched.  The same thing applies even at the centre of a black hole at the instant the event horizon forms.  There is always a still and forceless centre however small it is.

#### CliffordK

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #14 on: 20/03/2012 15:06:09 »
2.  You would hit the side of the hole.  At the equator you are doing at 1600kmph laterally, you would end up going faster than the hole... But if you were to dig pole to pole...

If digging equator to equator, you would have to design your tunnel with a curve due to the Coriolis effect.

#### syhprum

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #15 on: 20/03/2012 15:48:34 »
Soul Surfer

I find it hard to believe, if the hollow sphere was real thin walled and I was real long, not very realistic surely but would there still be no stretching ?
What I visualise is replacing the hollow sphere by its one dimensional equivalent i.e. two separated masses with a long me in between would I not be stretched then ?
« Last Edit: 20/03/2012 19:00:00 by syhprum »

#### imatfaal

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #16 on: 21/03/2012 10:42:47 »
Syhprum - if my maths is correct you cannot directly lower the dimensions; without redoing the maths I think the integration works because you are dealing with a shell in 3d with a inverse square law.  The size of the interior object does not matter - at every point within the shell there is no net force.  ie each subdivision of your long test subject will all feel the same attraction that is say none - therefore there is no variance in attraction (or direction of attraction) and no stretching or skewing

#### syhprum

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #17 on: 21/03/2012 19:07:22 »
Thank for the demolition of my hypophosis I have learnt not to argue with maths.

#### imatfaal

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #18 on: 22/03/2012 09:56:08 »
Thank for the demolition of my hypophosis I have learnt not to argue with maths.
Now you have said that I feel honour bound to check.  I am certain about the no stretching - but the lower dimensional model I am not so certain is incorrect. My maths homework has been set... :o

#### RD

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #19 on: 22/03/2012 13:47:00 »

#### syhprum

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #20 on: 22/03/2012 14:29:38 »
It would appear from the graph that a stationary long body at the centre would be compressed.

#### imatfaal

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #21 on: 22/03/2012 15:05:16 »
It would appear from the graph that a stationary long body at the centre would be compressed.
In a tunnel - yes.  in a shell - no

#### Soul Surfer

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #22 on: 23/03/2012 08:20:34 »
For an "infinite" uniform cylindrical gravitating body (or a very long one nowhere near the end or possibly a large torus stabilised by rotation)  there is no net force.  But I am not sure if it is a stable equilibrium and it might tend to break down int individual blobs.  It is interesting to note that in this case the gravitational field fall off as you leave the surface of the cylinder is not an inverse square law but an inverse first power law because of the dimensional reduction.

To take this one step further the gravitational field from an "infinite" massive plane does not fall off at all but is a constant.

This could have an effect on the models of the real collapse of material inside a rotating black hole towards the hypothetical ring singularity.
« Last Edit: 23/03/2012 08:22:43 by Soul Surfer »

#### Geezer

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #23 on: 23/03/2012 09:26:00 »
The whole thing sounds like a shell game to me.

Meanwhile, I'm trying to abolish "microgravity". Do I get any help? Not blinking likely!

"Ho no Mr Geezer. We're much too busy solving real problems to worry about silly stuff like that."

#### syhprum

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##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #24 on: 23/03/2012 19:37:03 »
How about abolishing macro gravity like the useful shields do in Sci Fi stories.

#### The Naked Scientists Forum

##### Re: What would happen at the centre of a tunnel through the Earth?
« Reply #24 on: 23/03/2012 19:37:03 »