what physical device can be used to construct a parabola?
i don't care what you're given... focus and directrix, maybe. preferably, though, the stuff we have is stuff we already know how to construct by physical processes and Euclidean constructions. one thing to make clear -- i don't want the parabola constructed as the locus of anything. i'd like to come up with some process by which an actual continuous curve is drawn on the paper, with hopefully as clear and simple a set of things to consciously worry about (once everything is set up) as for the examples i gave in the first paragraph.
i'd really like to know the answer... heh.