# The Naked Scientists Forum

### Author Topic: What happens if you drill a hole through Earth ?  (Read 19892 times)

#### cuso4

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##### What happens if you drill a hole through Earth ?
« on: 05/07/2003 19:47:21 »
Right, this is an interesting one.

Say if you drill a hole through the centre of the earth and drop a ball into the hole, what will happen to the ball?

I think some of you guys have heard this one before. And Tom, you can't answer because you know the answer already.

Angel
« Last Edit: 31/05/2004 05:35:34 by NakedScientist »

#### Donnah

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##### Re: What happens if you drill a hole through Earth ?
« Reply #1 on: 06/07/2003 01:39:59 »
Wouldn't it stop in the middle (gravity) and be squished by the pressure?

#### Quantumcat

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##### Re: What happens if you drill a hole through Earth ?
« Reply #2 on: 06/07/2003 05:43:05 »
The middle would have a little equilibrium spot where it would float as though no gravity?

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#### cuso4

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##### Re: What happens if you drill a hole through Earth ?
« Reply #3 on: 06/07/2003 09:33:12 »
Nope, they are not the answer I'm looking for. When the ball is dropped, it accelerates and then reaches a terminal speed at the centre. But the ball is still moving so will not stay in the centre.

Give it another try.

Angel
« Last Edit: 06/07/2003 09:56:02 by cuso4 »

#### Quantumcat

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##### Re: What happens if you drill a hole through Earth ?
« Reply #4 on: 06/07/2003 11:00:22 »
Okay, well the ball will accelerate towards the centre, keep going, be pulled back, probablt go past the centre then turn around, eventually ending up at an equilibrium in the centre. Is that right?

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#### chris

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##### Re: What happens if you drill a hole through Earth ?
« Reply #5 on: 06/07/2003 14:18:02 »
I'm with Quantum's reasoning on this one. I reckon that, assuming there are no 'losses' on the way as the ball moves (friction or air resistence) then it will accelerate all the way to the centre, reach maximum kinetic energy, then continue out the other side to the equivalent potential energy on the other side, and so on...bouncing back and forth along the length of the hole.

Chris

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#### nilmot

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##### Re: What happens if you drill a hole through Earth ?
« Reply #6 on: 06/07/2003 14:58:53 »
Yep, Chris you're right. My Chemistry/AG's Physic teacher told us this quiz, he actually used me as an example. When "I've" reached the middle of the Earth, there are no gravity acting on me the force is balanced. However I'll keep moving to the southern-hemisphere because all that kinetic energy don't just disappear (if we start from the north) and now head facing downward and going up if can imagin. Then gradually decelerating and reach the surface to only be pull back by the gravity towards the centre again.

It sounded quite painful when he said it.

Tom

#### cuso4

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##### Re: What happens if you drill a hole through Earth ?
« Reply #7 on: 06/07/2003 15:31:40 »
Well done Chris, the ball oscillates along the length of the hole.

Now, which is the most dense metal known?

Angel

#### chris

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##### Re: What happens if you drill a hole through Earth ?
« Reply #8 on: 06/07/2003 23:46:34 »
Er Iron is pretty dense...

Chris

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#### cuso4

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##### Re: What happens if you drill a hole through Earth ?
« Reply #9 on: 07/07/2003 07:46:47 »
Erm... yeah iron is pretty dense but it's not the most dense. Any other offers?

Angel

#### chris

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##### Re: What happens if you drill a hole through Earth ?
« Reply #10 on: 07/07/2003 10:01:31 »

"I never forget a face, but in your case I'll make an exception"
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#### Donnah

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##### Re: What happens if you drill a hole through Earth ?
« Reply #11 on: 09/07/2003 00:48:11 »
Osmium

#### nilmot

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##### Re: What happens if you drill a hole through Earth ?
« Reply #12 on: 09/07/2003 08:11:40 »
Wow, Donnah how do you know that? It is Osmium.

Tom
« Last Edit: 10/07/2003 10:56:17 by nilmot »

#### cuso4

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##### Re: What happens if you drill a hole through Earth ?
« Reply #13 on: 11/07/2003 07:13:28 »
I've got another question:

Space shuttles don't blast away vertically, they are projected at an angle like projectiles. Once the accelerator on the shuttle ran out, the shuttle starts falling to the earth. So why doesn't it hit the ground?

This is actually simple if you think about it carefully.

Angel

#### Quantumcat

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##### Re: What happens if you drill a hole through Earth ?
« Reply #14 on: 11/07/2003 11:32:23 »
The shuttle doesn't fall because it is blasted so far/hard that the earth moves a little in its orbit under the rocket. the rocket falls toward, but misses it. It gets pulled towards it again, the earth moves again, the rocket keeps circling and circling, trying to fall but just not being able to. If the rocked isn't blasted hard enough it won't go high enough to take longer and the earth won't move as far and it will catch the earth when it falls towards it. If it is blasted too hard the energy will take it away and it won't slow down in the atmosphere enough to have the energy taking it away be less than the diminishing gravity in order to be pulled back so it won't be pulled back, when it gets out of the atmosphere nothing will slow it down except the earth's gravity and the energy will be too much for the gravity to pull it back ... if that makes sense. I'm not sure about the blasted too hard one someone correct me if I'm wrong I didn't learn it just reasoned but it may be wrong. Everyone should know how things get into orbit! It's something you just pick up when young :-)

#### cuso4

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##### Re: What happens if you drill a hole through Earth ?
« Reply #15 on: 11/07/2003 16:16:03 »
Quantum, the shuttle would fall but because the Earth's surface is curved so the shuttle wouldn't hit the ground. The shuttle just keep on falling and yes orbiting the Earth.

Angel

#### cuso4

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##### Re: What happens if you drill a hole through Earth ?
« Reply #16 on: 15/07/2003 13:03:47 »

An astronaut was reparing a part of a satellite in space, he lost his connection (a rope say) to the space shuttle. All he's got are his tools, how can he get back to the shuttle?

Angel

#### chris

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##### Re: What happens if you drill a hole through Earth ?
« Reply #17 on: 15/07/2003 13:50:52 »
By chucking his tools away in the opposite direction to the space ship. The equal and opposite force he generates by throwing the tools will propel him towards the shuttle. That said, perhaps he'd better throw one tool at a time in order to make small corrections to his trajectory in case he goes 'off course' !

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#### Quantumcat

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##### Re: What happens if you drill a hole through Earth ?
« Reply #18 on: 16/07/2003 00:09:24 »
He could "swim" by moving his tool hard ibehind him, moving them slowly in front, then hard behind him again, that way he wouldn't lose his tools. If you sit in a chair that can spin, wave your arms really hard in the direction you don't want to go and you'll move in the other direction. Bring them slowly back and repeat. Just by swinging your arms you can make yourself spin right around. :-)

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#### cuso4

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##### Re: What happens if you drill a hole through Earth ?
« Reply #19 on: 16/07/2003 07:49:34 »
Well done Chris, that was the answer I'm looking for. But Quantum I think your method could also work too.

Angel

#### heythere

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##### Re: What happens if you drill a hole through Earth ?
« Reply #20 on: 08/08/2003 00:34:57 »
Hmmm what about the molten rock and metal that would get released while drilling the whole??? Wouldn't this fry the ball and probably the majority of the earth?[:p] lol
he he

#### esprit fort

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##### Re: What happens if you drill a hole through Earth ?
« Reply #21 on: 07/10/2003 18:16:27 »
quote:
Originally posted by cuso4

Well done Chris, the ball oscillates along the length of the hole.

Of course it would (assuming no friction etc., and pole-to-pole tunnel to avoid rotation/coriolis effects), but in what motion?

Well, 1 of the beauties of the inverse-square-of-distance in the law of gravitation is that all mass of a (sphere or) spheric shell can be thought of as concentrated in the centre, as felt by particles outside the sphere/shell, whereas particles inside the shell don't feel any net gravitation.

When our ball is at distance R from the centre, the gravitation force is proportional to
(mass closer to centre than ball) / R-square
Assuming uniform density: R-cube / R-square = R.
The force being proporional to displacement from equilibrium position, we have a harmonic oscillator. If a little friction is present, the motion will damp out gradually (exponentially), with a constant period.

quote:
Originally posted by cuso4
Now, which is the most dense metal known?

quote:
Originally posted by Donnah

Osmium

Osmium must be considerably more expensive than depleted uranium.
Uranium, being denser than lead, is sometimes used as ammo.
Depleted uranium is surely cheaper, and probably safer, than natural uranium, but it still causes political fall-out.

#### chris

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##### Re: What happens if you drill a hole through Earth ?
« Reply #22 on: 07/10/2003 18:57:03 »
Yep, the ball would exhibit simple harmonic motion wouldn't it ?

And about the political fallout - great line, I'll have to remember that. I think we can assume that the military have done their sums and come up with the cheapest answer, politically and literally.]

Chris

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#### cuso4

• Angel Delight
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##### Re: What happens if you drill a hole through Earth ?
« Reply #23 on: 18/11/2003 08:35:08 »
Hey guys, I got another good one. Have a go.

In my physics lessons, I've been learning about gravity. The gravitational attractive force becomes less as you move away from the Earth's surface. So does this mean as you approach the Earth's centre the gravitation pull becomes bigger? Please explain.

Angel

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#### Quantumcat

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##### Re: What happens if you drill a hole through Earth ?
« Reply #24 on: 18/11/2003 08:53:26 »
I suppose yes, that is right. And there is an equilibrium when you are an infitessimally small (how to you spell that?) point in the very centre?

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#### The Naked Scientists Forum

##### Re: What happens if you drill a hole through Earth ?
« Reply #24 on: 18/11/2003 08:53:26 »