Wittgenstein's rod

Wittgenstein's Rod is a thought experiment attributed to Ludwig Wittgenstein.

A rod passes through a sleeve, in which it can smoothly slide. The sleeve is fixed to a wall, forming a pivot. One end of the rod is moved in a circle. What shape does the other end of the rod describe?

The seemingly obvious answer is a circle, but in fact it describes a cardioid.

External links

• Geometer's Sketchpad applet