The 42 minute gravity train

This factoid is a strange hypothetical one involving gravity so bear with me.

If you were to dig a tunnel from anywhere on the Earth's surface, right through the Earth's core to the other side, and then using gravity alone, travel through. You would at first accelerate (reaching a top speed of approx 11,000 meters per second as you passed through the core) and then decelerate (using up the kinetic energy you had gained to overcome gravity pulling you back) coming to a stop neatly at the other side.

This 12,700km journey would take you precisely 42.2 minutes.

Now dig another tunnel linking any two points on the Earths surface, say New York to London. Now that tunnels going to be nowhere near as long as the one going all the way through, because the distance between New York and London’s only about 5,500km.

This 5,500km journey would take you precisely 42.2 minutes.

No matter which points on the Earth’s surface you pick, 42.2 minutes. The shorter the length of your tunnel, the slower you will travel (because the earths gravity’s will be less) ending up at your destination 42.2 minutes later.

This hypothetical idea of an object accelerating inside a planet was first proposed by Robert Hooke in a letter to his great rival Isaac Newton in the 17 ^th Century and still puzzles Physics students and science fiction enthusiasts today.