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Disc rolling inside a Disc

A GOLD disc rolls without slipping inside the edge of a SILVER disc. If the discs are the same size, how many revolutions does the GOLD disc make as it rolls around the inside of the SILVER disc exactly once? You may find the result surprising? Can you explain it? What happens if the discs are not the same size – if the radius of the GOLD is half that of the SILVER? one quarter? one third? What questions could/would you ask of your students based on this applet?