This lecture covers the relative strain energy in torsion for circular and non-circular bars, the concept of shear stress generating torsional stresses, the relationship between applied load, shear stresses, and relative deformations, the determination of shear modulus through experimental tests, and the torsion formula for calculating shear stress. It also explains the relative torsional strain, shear stress distribution, internal moment equilibrium, and torsional shear stress for circular and thin-walled tubes.