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

Ignoring all the laws of physics (which is what makes SF a possibility for me - I'd rather go to impossible places in my imagination and stick to reality when I'm awake) if you you into the hole head first you'll come out head first, hence "up". If you go in feet first, it'll be a breech birth and you'll come "down"

So, if you're defining 'up' as coming out of a hole head first, then if I dive headfirst through a hole in a platform 100m above the ground will I still be travelling 'upwards' when I hit the ground headfirst?
I think up or down are defined according to relative position of thing x and surface of the earth. Hence you'll be travelling upwards when you shoot out of the other end of the centre-of-the-earth tunnel regardless of whether you arrive headfirst or breech.

Defining 'up' and 'down' using the surface of the earth as a reference point is like sooo 19th century, guy .

I prefer down being the direction which the net effect of gravity acts on your body.
Thus, assuming a tunnel through a totally solid planet with no air resistance and no bumping into the walls etc. , if you jump in feet down by the above method and pass through the centre of the earth without rotation you will emerge feet up! Now that's magic .

Its not likely you would emerge from the other side.. unless the gravity of the earth was not consistant.. even with no air in the tunnel your acceleration going down would be cancelled going up.. you never get out more energy out of a system than you put in.. its always less.. with a vacuum in the tunnel you would oscillate back and forth many times no doubt.. but eventually you would wind up in the middle.. no matter how you look at it the answer is if you fell into a hole going thru the earth.. you would wind up at the center of the earth in the deepest possible hole.. eventually the energy would wind down.. it has to... its an interesting thought experiment though.. im not sure what part the rotation of the earth has to play.. if any...

ive got an interesting perspective. if gravity is like a magnet, pulling harder on an object the closer it gets, and earth had no mass to crash into then it would suck all that passed by to the center point inward spiral style. the center would have to be many times more powerfull than the crust of matter it has collected around it. like a magnet.

If there was no friction at all (i.e a perfect vacuum) then there would be no energy lost from the system...so, starting from just above the surface of one edge of the planet you would accelerate towards the centre then when you pass the centre you would be decelerated by gravity until you came to a stop...exactly the same distance outside the other edge whereupon the whole thing would happen again.

Obviously the tiniest interference in the closed system above, by friction or the tidal pull of the moon etc. would disrupt this and you would end up in ever decreasing oscillations, eventually settling in the middle.

Now, if we have a rotating hole it becomes more complex altogether...I shall have to think about this factor.