This is a problem that appears in many calculus texts. The problem is that of girding the equator of the earth with a belt, then extending by one unit (here, taken as the foot) the radius of the circle so formed. The question is by how much does the circumference of the belt increase. This problem usually appears in the section of the calculus text dealing with linear approximations by the differential.

This app is also the subject of a blog post on MaplePrimes: Girding the Equator of Earth with a Belt