Consider a spherical ball on a plane and a point marked on it. We want to roll the ball on a closed polygon of the plane to bring the mark to the top of the ball while the ball is on its first place. Note that the ball must not roll in its place (it means rolling without moving on the plane). Prove that this is possible.
Solution
Let r be the radius of the ball and O be its first position on the plane. First, we roll the ball (sphere) on its great circle that passes through the marked point and the top point of the ball, until the mark be on top of the ball. Let A be the current position of the ball on the plane. It is trivial that OA<2πr. So there exists a point B on the plane so that AB=OB=2πr. Now we roll the ball from A to B on the segment AB, and since AB=2πr, the mark will be at the top of the ball. Then we roll the ball from B to O on BO and the mark is at the top of the ball while it is on its first position.
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Source: MathNet,
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