3550.11 – LP Phonograph Record

Tags:Problem Set 99Geometry-Euclidean

Consider a 12-inch diameter LP vinyl phonograph record. The diameter of the unrecorded part in the middle is 4 inches. The outer edge has 1/4 inch of unrecorded space. There are about 90 grooves per inch. If you put the needle down at the beginning of the record and let the whole side of the record play, how far will the needle travel?


Ha this is practically a trick question. The needle doesn't go round and round--the record does. The needle just moves slowly from outside to inside, a distance of 3 3/4 inches. (Actually, if the needle is on a pivot it swings in an arc, so it'll travel a teensy bit more than 3 3/4 inches.)