[Yair Doza (OP)]

Hi

My 11 years old son asked me: ' If I drop a marble through a hole in the Earth (a hole that goes all the way to the other side) where will it stop? Where will it?

Thanks.

[CPT ArkAngel]

If there would be no air, the marble would fall and accelerate until it reaches the middle of the earth. Then it would slow down until it would stop on the other side. Then it would fall back to your side in the same manner in a perpetual motion.

In reality, the air would slow it down by friction and the marble would never reach the other side. The oscillation would be shorter for every motion back and forth until it would stop in the middle of the earth where there is no acceleration due to gravity.

I neglected the fact that the earth rotates. The earth rotation would cause the marble to hit the wall in a complexed pattern depending on where you dig the hole... To minimize it, the best would be to dig a hole from the north geographic pole to the south geographic pole.

[RD]

If the Earth was solid, (rather than having a hot soft center), and a hole was drilled all the way through, it would fill with air. The air-friction would slow the marble so it wouldn't overshoot the center by much.  [ Collisions with the tunnel wall would also slow the marble down even further ].

[rmolnav]

That question has more recently been discussed, with many more replies than here:
https://www.thenakedscientists.com/forum/index.php?topic=68888.new#new