This lecture covers the resistance of soils and shear testing, discussing common simplifications in geomechanics, the concept of plasticity in materials, and the physical causes of plastic phenomena. It also explores the three phases of elasto-plasticity, mathematical approaches in geomechanics history, and classic failure criteria. The instructor presents various theories and models, such as Von Mises and Drucker-Prager, to analyze soil behavior under different loading conditions.