This lecture covers the concept of conservative forces and energy conservation, explaining how all forces are conservative when no work is done, and how the work done by a force is path-independent. It also delves into the relationship between conservative forces and potential energy, emphasizing that the difference in energy at two points is equal to the work done. The lecture further explores the calculation of potential energy and the equivalence between conservative forces and non-working forces. Examples are provided to illustrate the application of these concepts, such as the energy of a pendulum and the work-energy principle.