The lecture covered potential energy and the conservation of energy. Key points included: 1) Work done by conservative forces like gravity is independent of path. Potential energy U can be defined as mgh for gravity and 1/2kx^2 for springs. 2) The work-energy theorem states the work done by non-conservative forces equals the change in kinetic and potential energy. 3) For problems involving gravity, conservation of energy can be used to calculate changes in speed and height by setting the initial gravitational potential energy equal to the final kinetic and potential energies.