Question: if two people dug a hole, one in Australia and one in England. then they both jumped into their separate holes, gravity will pull them. but if those two people jumped and kept digging and digging and they dug so much, they met in the middle. what would happen?

David Wilson answered on 18 Nov 2014:

This is actually set as a test question in physics degree courses! If you were to dig a hole right the way through the Earth, passing through the core, and jumped, then the following would happen (neglecting air resistance, let’s say you can pump out the air in the hole and wear a space suit.)
1. As you fell towards the core, you would accelerate and get faster (gravity)
2. As you got close to the core the amount of the Earth between you and the core would get smaller, so there would be less gravity and you’d accelerate less- but you’d still be getting faster.
3. As you passed through the core you’d be going really fast. When you get to the half way point, the gravity would switch direction and you’d start slowing down.
4. You’re still going really fast though, so you’d keep heading away from the core. As you got closer to the end of the tunnel the gravity pulling you down would increase, so you’d slow down more quickly.
5. Here’s the neat bit: you’d slow down so that you’d stop exactly as you reached the other end of the hole! The whole system is symmetrical.
6. Then you’d fall back into the hole and the same thing would happen in the other direction. Eventuality you’d end up at the top of the hole where you started. Better grab on to something, or you’d end up going back and forth through he hole for ever!
7. the whole trip, there and back again, would take almost exactly 42 minutes. If you do A-level physics, you will be taught what you need to work this time out. If you can’t wait, look up Simple Harmonic Motion.