This lecture covers two famous experiments in mechanics: the recoil effect and distinguishing between external and internal forces. It also delves into the study of coupled harmonic oscillators, explaining the symmetrical and antisymmetrical modes of oscillation and their mathematical representations. The instructor demonstrates a spectacular experiment involving creating a vacuum in a tube and launching a ping-pong ball at high speed. The lecture concludes with a slow-motion analysis of the collision between the ball and a membrane, showcasing the impact dynamics.