Under the action of horizontal tensile force F, the force arm changes when the iron bar rotates counterclockwise from the vertical position to the horizontal position with point O as the rotation axis

Under the action of horizontal tensile force F, the force arm changes when the iron bar rotates counterclockwise from the vertical position to the horizontal position with point O as the rotation axis. The arm of force is the vertical distance from the fulcrum to the line of force action. Therefore, the arm of the bottom force is O 1, and the arm of the top force is 03. Therefore, the variation during the counterclockwise rotation of the power arm from the vertical position to the horizontal position is reduced.