From NOVA Science Now:
Ever wonder what would happen if you dug a whole from one side of the Earth, through the center, out the other side, and then jumped in? Watch NOVA scienceNOW host Neil deGrasse Tyson take the plunge.
Watch all new episodes of NOVA scienceNOW every Wednesday night this summer at 9pm on PBS, starting Wednesday June 25. Learn more at: pbs.org/novasciencenow
Julian, the reason the pull of gravity lessens as you fall toward the center is because a portion of the earth is now above your head, and is pulling up on you to counter the mass pulling down on you. Once you hit the middle, it's pulling on all sides of you equally, cancelling itself out. As your momentum keeps you going past that point. Yes, the law of gravitiy is based on the square of the distance between two masses and thus gets stronger the closer you get to the earth, but once you are inside the earth you can no longer treat it like it was one single point in space concentrated at its center. That mathematical simplification of reducing the entire mass of the earth down to a single point stops being valid once you are inside the mass in question.
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