Scott Russell linkage
Consider a linkage of the configuration according to geometric mode below. This mechanism, called the Scott Russell linkage (see https://en.wikipedia.org/wiki/Scott_Russell_linkage), the principle of which is used, for example, in some kinds of car jacks, consists of two bars, labeled AC and BP in Graphics below, so that |AC| = |BC| = |CP| = a. The bars can pivot about joints C and A, the latter attached to the base p (the x-axis in figure below).
The task solved in this applet is to find the locus of point Q, the arbitrary point between C and P, when B is moving along p.