First, if you are around Edinburgh, I am giving a talk tonight at the International Magic Festival on the psychology of magic and illusion. Details here.
Second, on Friday I set this puzzle…
A long playing record has a 12 inch diameter. The unused bit in the middle is about 4 inches in diameter, and there is a smooth outer edge one inch wide around the recording. If there are 91 grooves to the inch, roughly how far does the needle move during the recording?
If you have not tried to solve it, have a go now. For everyone else the answer is after the break.
About 3 inches – can anyone explain why?
I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.