Suppose you have a uniform beam, such as a piece of plywood that is 4 feet long, six inches wide, and half an inch thick. Suppose you cause it to bend by attaching, for example, a string under tension between the two ends, similar to how one would string a bow for shooting arrows. What shape will the beam take?
My goal is to create a parabolic shape, for use in a trough-type solar concentrator. Ideally, I would like to be able to vary the focal length simply by changing the tension on the string. I want quite a long focal length - several times the length of the beam - so the beam will not be bent very much.
If the natural bending shape of a uniform beam is not a parabola, what profile could I use when cutting out the plywood so that it would take on a parabolic shape (see the bottom three profile shapes as examples)?