This document discusses the fundamentals of mechanics of elastic materials. It defines key terms like stress, strain, and modulus of elasticity used to characterize how materials deform under force. Stress represents the force applied over an area, strain is the deformation normalized by length. Modulus of elasticity relates stress and strain and indicates a material's stiffness. Examples show how to calculate stress, strain, and modulus of elasticity for an elastic rod under tension. The stress-strain curve is also explained, distinguishing elastic deformation, permanent plastic deformation, yield point, ultimate strength, and failure point.