Puzzle 14 Coinundrum.pdf

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Answer

The coin will look like this. Option B.

Demonstration:

Explanation

Most people would say that the coin appears upside down compared to its starting orientation. This is because the two coins are similar in size, meaning they have the same circumference. Hence, if one rolls around the other, the rolling coin should cover the entire circumference of the fixed coin with just one full rotation. Therefore, the coin should appear upside down at the "?" position, or the halfway point.

In reality, this is reasoning is wrong. The rolling coin actually needs to make two full rotations to complete one journey around the fixed coin.

I would love to make a better explanation, but this subject is really hard to explain without animation, which I cannot do.

Check out this Veritasium video for detailed insight. The question is slightly different but it is the same paradox.

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