Reconsider the angular-momentum balance of Fig. 4.5 by adding a concentrated body couple Cz about the z

Question:

Reconsider the angular-momentum balance of Fig. 4.5 by adding a concentrated body couple Cz about the z axis [6]. Determine a relation between the body couple and shear stress for equilibrium. What are the proper dimensions for Cz? (Body couples are important in continuous media with microstructure, such as granular materials.)

Reconsider the angular-momentum balance of Fig. 4.5