Take about 2 meters of plastic hose pipe and stretch it straight.The function plotted in Geogebra (L = 10)You can build this in Algodoo by putting motors between lots of sticks, see if you can build one like mine.Plot the PE in the spring that joins the ends.There are a couple of useful tricks you need to know.You can also try building this in Geogebra (a bit more tricky).