Question: If there was a tunnel that went right through the centre of the Earth (and was safe, no magma) and I jumped down it what would happen. Would I stop immediatly in the centre, or would I go past the centre, rise and then fall past the centre and rise and fall and so on?

Daniel Scully answered on 17 Nov 2011:

Gravity always accelerates you towards the centre of mass, which in this case is the centre of the Earth. The ammount by which it pulls you, depends on the amount of mass between you and the centre (so the stuff behind you doesn’t have an effect)

So, as you (very bravely) jumped down the hole, gravity would accelerate you towards the centre of teh Earth, and you fall faster and faster until you reached the centre.

At the centre you’ve reached your maximum speed. At the centre there is no gravitational force on you at all, but you’ve been accelerated to high speed from the fall so you’ll keep on going past the centre.

Now though, you’re travelling away from the centre (towards the other side of the Earth) but the centre of the Earth is behind you. Gravity tries to pull you towards the centre so the force of gravity will start slowing you down.

In fact, assuming the surface of the Earth on either side of your hole was at the same height, it would slow you to a halt exactly as you reached the other end of the hole.

Hopefully someone there would grab you, else you start falling again, and would be doomed to fall back and forth through the Earth until someone caught you.

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That answer only works though if there is no air in your hole. If there was, friction (or air resistance) would slow you down and you wouldn’t have enough speed to reach the other side of the planet. Eventually after falling back and forth a bit, you’d end up at the centre of the Earth. Stuck.

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The first answer also assumes the Earth isn’t spinning. If you dug your tunnel from the north pole to the south pole you’d be fine. But if you dug it anywhere else, the spin of the Earth would bash you into the sides of the hole. Ouch.