This lecture covers the concepts of stresses, strains, and deformations in 1D and 2D structures, including the reduction of 3D problems to 1D. It also explores the effects of temperature on material properties, leading to thermal strains and stresses. The equilibrium equations, Hooke's law, and the treatment of bodies as springs are discussed, along with examples of axial deformation in loaded bars. The lecture concludes with the resolution of hyperstatic systems and the determination of reactions in constrained structures.