After a short rehearsal of the previous module, the Lagrange equations of motion are discussed. It is shown how to derive these equations using the energy principle and the generalized forces, as discussed in Module 2. As an example to the Lagrange equations, the pendulum is discussed. It is shown how the phase portrait of the system can be derived. In addition to the conservative forces and conservative systems, the Jacobi energy function is introduced.

One sample problem is given in the module and can be found in the exercises of this module.