Problem 16.11: Dig a hole through the center of the Earth

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A very wealthy individual proposes to dig a hole through the center of Earth and run a train (the small black circle) from one side of Earth to the other, as shown in the animation (position is given in Earth radii and time is given in seconds). Which of the animations correctly depicts the motion of the train? Ignore frictional effects and treat Earth as a uniform mass distribution. Restart.