h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Imagine that you tunnelled right through the Earth - from one end to the other side. I know that it would be impossible - with the Earth's core and everything, but if you COULD and you jumped into a hole at one end... would you come 'up' out of the other side? Or would gravity mean that you bounce from end to end, eventually stopping in the middle?
Just a thought.

Air resistance would slow you down much too much to reach the other side. If you pumped all the air out of the hole, you might make it. However, you'd be moving pretty damn quickly when you reached the centre, so if you happened to graze against one of the walls...

A _very_ rough calculation (v^2 = 2as, taking 0.5g as average acceleration over the 6000km drop) gives a speed at the centre of 7750m/s, or 28000 kph. A correct answer would require calculus, but I think I'm within a factor of 2 or so. It's way more than fast enough to hurt, anyway, so who cares?

gravity is a property of mass right ? the weight of the earth crushes it into a ball. pulling everything toward the center. if you had a hole in the middle of that ball what would gravity be like there ? all the mass would be pulling away equally in all directions . weightless ? it would be strange to walk on the walls inside of a sphereical chamber . gravity would only be a fraction of what it is normally i would think .. or would it ? it sounds like a great place to play a game of hockey though.. sphere hockey .

Okay I am only in the high school so I am not gonna lay down calculus and I can only guess a few things. Like if the core was hollowed out it would instantly collapse wouldn't it. I mean all the walls would be pulling on eachother and there is so much mass. But I am not sure how much gravity would affect a person as they flew through the center and were able to miraculously survive. Would it be enough to make them not reach the other side of the earth or would it just slow them down when they shot out the other side? beats me

Yes. Sod the pressure/heat/general improbabilty of the thing.
Let's hollow out the centre of the Earth and play hockey.
Of course, the rules may need to be changed...

I did not write the above. For one thing, I would not use the second word in the above. Someone appears to have logged on as me somehow. I am NOT happy

But say you disregarded the gravity thing. If you jumped feet down into the hole, which way up would you come out?

Assuming that the air is pumped out of the hole, you don't hit the sides and the gravitational pull doesn't rip you apart as you travel through, in theory you should exit the hole at the same speed you entered it, the braking force from the gravity counterracting the acceleration you would feel in the first place.

You would exit feet first as well (unless you twisted midway through).

If there was any other braking effect, eg air-resistance, you would probably end up in the middle.

The gravity shouldn't rip you apart, as it would decrease from the surface level to zero as you approached the middle, and increase again to the usual level as you approached the surface on the other side.

Whatthe?!
It was ME who wrote that!
I don't know what has happened... I promise I haven't logged on as you or anything.
I didn't realise that people took offence at the 's' word... sorry if I caused any.
Big Mad Mr T. (Just in case it messes up again).

Okay, never mind. I just wonder how that happened? I didn't even find this forum until "I" posted there.

as for the question - it was actually on one of my physics exams.

if you do an integration, the gravity at any point is the gravity of the shell at that radius. If you take a drill and drill a hole through a perfect earth-sized sphere, a pop a ball bearing in, it should oscillate back and forward along the hole. Of course, the earth is not solid inside and also is incredibly high pressure, and is not round, so non of the above is feasible. hmm... I think I'll go look out my notes on that subject...

Well, one explanation is that I may have composed my reply offline - as I sometimes do - and then re-connected and posted. But I don't recall doing so.
When else have you been misrepresented?

As for the tunnel thing... well the whole thing's so thoroughly implausible who can really say?
I suggest we have a go with the moon as a test-run... does it have a molten core? Plus it's less dense.

As I understand it gravity is the attraction of all matter to all other matter, but decreases rapidly with distance. The only reason we can stick to the Earth is because it is so massive ie has an awful lot of mass.

In theory at the centre of the Earth, all cells in our bodies would be attracted in all directions by the billions of tons of rock around them. As that force would likely be stronger than those forces holding our bodies together due to our proximity to the mass, we would be ripped apart. The sum of the forces would be zero but that does not stop something still happening to the object they are acting on.

Consider being deep under water, the sum of the pressure on the body doesn't move it but they all act in on it to crush said body because there is little resistance to them. Similarly the forces acting at the centre of the Earth are unlikely to have much resistance to pulling the body apart.

Assume the earth is a perfect sphere.
You are at the exact centre.
To your left you have mass M at distance x
To your right you have mass M at distance x
therefore
To your left, you have a force F attracting you leftwards.
To your right, you have a force -F attracting you rightwards.
The forces sum to exactly zero
Therefore there is no net force acting on you in the left-right direction.
Turn around, the same is true

There is no net force acting on you.
You are not ripped apart by the forces.

(I won't answer about falling apart due to lack of forces)

I was about to argue with that, but thought a little harder and it makes sense.

My ripping apart theory only holds if the forces act only on the edge of the body, not right the way through. Faulty reasoning on my part obviously. I'm glad this wasn't an exam question.

I'm glad I didn't get involved in this thread. When I first read the question I thought we would be arguing about linguistics ("is it up or down?" seemed like a language puzzle or possibly logic. If I _had_ answered I would have been crushed beyond death by now by all the force of argument from these physicists.

Hey - who are you calling a physicist?
I'm an Engineer, if you _don't_ mind

sorry

Likewise.
(I think the answer's "up")

ignoring a shpere shaped hockey arena at the core for the moment..that was a thought experiment and a truly neat idea for a sport..and a way of thinking of gravity in reverse . pulling away from the center instead of towards it . assmuming just a straight hole thru the earth ignoring the enormous pressures and heat involved.. as you fell toward the center true gravity would cancel from all around but it would cancel most in the exact center of the tunnel.. anywhere but the exact center one side or the other would pull more.. therefore i believe that as you fell. unless you could manage to stay in the exact center of the tunnel.. you would be drawn toward one side or the other.. and eventually come skidding to a stop near the center.. it would appear as a tunnel to you but one where you could walk on the ceiling or walls just as easily with a force acting on objects in the center tending to hold them there.. until tiny turbulence throws them one way or the other.. a truly strange place.. and getting out would be extremely difficult.. as you would be at the bottom of the deepest hole possible .