Whirl the stopper and washers overhead. The tension in the string should keep the hanging mass suspended. Pulling down on the mass will bring the stopper into a tighter circle, reducing the moment of inertia and increasing the angular velocity. Because the angular momentum [which is equal to m*r*r*(omega) or m*v*r, since v=r*(omega)]is approximately constant, the tension in the string (centripetal force=(m*v*v)/r) also rises.