# Digging a hole to through the Earth

Suppose you dug a hole straight down all the way through the Earth and out the other side. Take away the heat, magma, etc. If you dropped a ball into the hole, would it come out the other side?

Dunno. I started this experiment in my back yard about 40 years ago, but I never actually dug through to the other side. I’m about .001% into the project. I’ll get back to ya.

That’s still very impressive, since you’ve managed to dig a mine three times deeper than any other.

Can I come over and play?

when you get to the middle of the earth, there should be a set of keys and a wallet. I know they are there, I’ve looked everywhere else. Can you bring them back for me?

Actually, RickJay, I only dug about 4 feet deep. The rest of the time I was…researching. That’s right…I was researching the project. And shoving my little sister into the hole.

Boo, about those keys…When I was down in the hole, I saw some Chinese children running away with something. Might have been your keys.

I remeber as a four year-old I made several attempts to dig to Austrailia from my back garden, never got further than about 10 inches.

I’ve not read the column, but what would happen is that the ball would necessarily lose some of the energy it ganied from falling (quite a large amount of it if there is air in the hole) so it could never make it quite as far as the other side.

OK, so now I’m thinking, that when the ball reached half way, would it not now be going up?

Hot diggity Ma! It’s a hole diggin’!

And being the stereotypical male, when can I spit down it?

Of course. In fact, in the physics class I taught, I noted that “down” = “towards the center of the Earth”.

Three times? Really? What’s .001% of the Earth diameter, about 4200 feet? So, a third of that is 1400 feet, is that really the deepest mine? I thought they went down miles. The Grand Canyon is a mile deep, right?

Sorry, I got distracted there.

That’s an interesting question. I did some Googling and found that the equatorial diameter of the earth is ~12,756 km(41,850,392 feet). The deepest hole ever dug is one near Muramansk, Russia:

So, from there we have some simple math to do:

Kalhoun’s hole was 4 feet / 41,850,392 feet = .0000001%
The Kola Dig was 40,000 feet / 41,850,392 = .001%

Either way, a piddling amount of the way.

Sorry, of course you need to move those decimal points over a couple places to get the percentages. I blame the beer.

OK, I’ll ask it now, if I’m ever going to…

After you got past the sea of magma (A minor detail, that), would the actual CORE of the planet be solid Iron, or liquid Iron?

And what color would it be?

The Earth is known to have a liquid core about half way down–but sometime after that was discovered, we found that inside that liquid core, there seemed to be a solid inner core. So now we say that it has a liquid outer core, and a solid inner core. Beneath the solid crust, which has cooled, is a solid mantle, which is nearly liquid in a zone just under the crust.

If the earth was uniformly spherical and was not rotating and the tunnel was completely straight and passed directly through the middle and the whole thing happened in perfect vacuum, then an object dropped into one end of the hole would fall, accelerating until it got (about)halfway, then it would rise through the other half, decelerating all the way, to emerge at the other end - assuming you dropped it into the hole from shoulder height, it would come up to shoulder height at the other side, then fall back in and do the same in reverse, ad infinitum.

But…

The Earth is not uniformly spherical, so you might drop it from a valley at this end - it would not build up sufficient velocity to rise to the top of a mountain, where the tunnel emerges at the other end.

The Earth is rotating - unless the tunnel followed the axis of rotation, coriolis effect would cause it to strike the wall, slowing it down and thus preventing it from building up the required velocity to overcome gravity on the way back up on the second half of the trip

If the tunnel was not completely straight, the object could strike one of the walls.

If the tunnel did not pass through the exact centre, the object would fall towards the exact centre and would strike one of the walls.

The presence of atmosphere would prevent the object from accelerating indefinitely - it would reach terminal velocity and woulc not be able to complete the second half of the trip.

The Earth doesn’t have to be uniformly spherical. It’s sufficient for the two ends of the tunnel to be at the same gravitational potential.

but how long would it take for the ball to reach the end? that is, of course, it was a perfect situation (vacuum, straight tunnel etc.)

Sure - it was something of a simplification - uniform (or at least symmetrical) density might be important too.

Okay. I just wanted to point it out, because the Earth, in fact, isn’t uniform, but that wouldn’t stop it from working.