The differential equations governing motion in dynamic systems can be derived in a number of ways, the result however being the same. The most common way is through use of the equations of dynamic equilibrium of all forces acting on the dynamic system, known as D’Alembert’s principle, which derives from Newton’s second law. Two energy methods can also be used, one based on the principle of virtual work and another based on Hamilton’s principle. Their use depends on the problem at hand: for instance, Hamilton’s principle is well suited for complex dynamic systems, while D’Alembert’s principle is more conveniently applied to the study of discrete dynamic systems modelled with relatively few DOF. An application of the aforementioned three methods will be done here in reference to the simplest dynamic system, namely the SDOF oscillator.