The global balance of momentum and the equation of elasticity. The Cauchy formula and the Neumann boundary conditions. Energy and the constitutive relations of elasticity and thermoelasticity. Constitutive relations for viscoelasticity and 1-d elastoplasticity. The initial-boundary value problem of elasticity. The virtual work principle and the weak formulation of elasticity. Extrema of the energy functional, Ritz and Galerkin approximation methods.