As shown in the figure, a uniform rod can be rotated freely about point A in the vertical plane. When a vertical upward force F lifts up the rod slowly from position shown, which of the following statements is correct till the rod becomes horizontal ?

Question

G1 Physics · Accepted Answer

During slow lifting, angular acceleration is negligible, so the rod is in rotational equilibrium about A. If θ is the angle the rod makes with the horizontal, torque balance gives F(L cosθ)=Mg(L/2 cosθ), hence F=Mg/2, a constant. As the rod moves from the shown inclined position toward horizontal, cosθ increases, so the moment FL cosθ increases. Therefore option A.