This classroom-tested text incorporates the variational formulation of mechanical models based on the principle of virtual work (PVW). All models are presented following the same sequence of steps, including kinematics, strain measurement, rigid body deformation, internal and external work, equilibrium, constitutive equations, and structural design. This presentation allows readers to establish a logical sequence for treating any mechanical problem. The author offers step-by-step explanations of the limitations and strengths of mechanical models and includes many real-world examples. MATLAB#xAE;programs are supplied to run the examples.