Example: applet to explain `springs' used in membrane model.
Here the user may move either the mass or one of the graph tracers to plot out the force-extension and energy-extension graphs for a simple spring obeying Hook's Law. We have purposefully made the inteface very simple with buttons that clear the graphs, draw in the whole graphs and offer help (including expanation of the other two buttons). The help information appears in a pop-up window.