$4.$ The stiring rod total length is $2$L and diameter is D$,$ it have half part in the liquid$($as shown in the picture$),$ at the top of the rod act a concentrated couple m$.$ If theresistance moment of the liquid to the rod is proportional to the depth, the lowest endresistance moment set is to$,$ the Shcar modulus of the rod is G$.$Request $(1)$: request the maximum shear stress in the cross section of stirring rod.Request $(2)$: request the Relative rotation angle of two ends of stiuring rod.to