The document discusses constitutive equations and materials behavior. It provides the following key points: 1. Constitutive equations describe the relationship between stress and strain for a material and can distinguish between solids and fluids, as well as different types of solids. 2. For elastic solids, stress is proportional to strain, following Hooke's law. The proportionality constant is the elastic modulus. Materials exhibit linear or non-linear elastic behavior. 3. Isotropic materials have properties that are identical in all directions, allowing their behavior to be defined by just two independent constants in the constitutive equation.