Michael Bennett, of Dunedin, asks :-
If we drilled a hole right through the Earth and dropped a two-tonne lead weight would it go right through or stop at the middle?
Craig Rodger, a physicist at Otago University, responded.
If we could drill right through the Earth, and if the amount of friction was low, it would stop briefly on the other side then fall again, oscillating about the centre of the Earth.
When we release the weight it falls down the hole, getting faster and faster until it passes through the centre. Then it slows until it comes to halt somewhere on the other side depending on the amount of friction.
If no-one catches it, it will fall back towards our side! This is because of the conservation of energy. When it starts, and when it stops on either side of the Earth, all its energy is potential energy because it has no motion but is attracted by the gravitational force to the centre of the Earth. At the centre it has no gravitational (potential) energy and all its energy is kinetic, i.e., due to its speed. Anywhere else it has a combination of potential and kinetic energy.
This is really the same physical situation as a pendulum. If there were no friction and no air resistance, the weight will keep oscillating back and forth between the two surfaces.
So, why don't we use this as a cheap way to go to Spain? For a start the Earth is around 13,000 km across, and the deepest hole ever drilled is just 12 km deep, and that took twenty years to drill! The costs get higher and higher as one tries to go deeper. Also, the interior of the Earth is mostly liquid and under very high pressure. Around 200 km below the surface, the temperature of the rock is already near the melting point of rock. Its easier, and cheaper, to fly around the world.