What would happen to a ball dropping through the centre of the earth?
If you could drill a hole from the North Pole to the South Pole, and it was big enough to drop a stone through, would the stone go all the way through and out the other side and keep going? Or would it just go to the centre and stop? Or would it go half way through and bounce backwards and forwards and keep going backwards and forwards forever and ever?
It would depend on whether you took all the air out of the tube first, because if there was air resistance it would drop all the way down. It would be going quite fast at the bottom but probably not very fast because it would be losing lots of energy to friction. It would go over to the other side a bit, but then it would start falling in the other direction and it would start coming north again, and it would oscillate and eventually end up in the centre of the earth. If you took out all of the air from the tube, it would drop all the way through the earth and end up pretty much at the South pole, it would stop there and it would come back again. It would go backwards and forwards and would look like simple harmonic motion, a bit like a pendulum. It would take about 90 minutes to bounce back and forth. If you imagine that you don't take the air out, it's worth thinking about water as an analogy. Every time you go 10 metres underwater, you increase the pressure pushing down on you by 1 atmosphere. If you were to go all the way to the centre of the earth pretty soon the air would become so dense and there would be such a huge amount of atmosphere above you, it would be like trying to swim through a solid. Which is why the earth's core is solid; because of the huge pressure that there is down there. The thing is, you wouldn't be able to go very very fast because there would be this tremendous resistance. You'd actually fall probably forever, because you'd go so slowly.