if you fell into a tube through the earth

[mumblefuzz]

Cecil says "I was going to calculate how long this would take, but twenty minutes of computation has produced no useful result."

This came up in a physics exam I wrote ages ago.

As it happens the answer is the same as the period of a satellite in orbit at the same altitude -- in this case zero, sea-level. So something less than an hour and forty minutes if I remember correctly.