Explanation: Controlling force curve: When the graph between the controlling force as ordinate and radius of rotation of the dead weight balls as abscissa is drawn, then the graph obtained is called the controlling force diagram. Controlling force, Fc = mω2r. The relation between the controlling force (FC) and the radius of rotation (r) for the spring controlled governors is given by the following equation. FC = a.r – b when FC = ar – b (governor is stable) i.e. B FC = ar (governor is Isochronous) i.e D FC = ar + b (governor is unstable) i.e F [ alt="contolling12" src="//storage.googleapis.com/tb-img/production/18/04/contolling12.PNG" style="height: 320px; width: 300px;">