This lecture introduces the control volume approach in fluid dynamics, focusing on the Reynolds transport theorem to rewrite physical laws for a fixed volume. The instructor explains the importance of mass conservation, Newton's second law, and energy conservation in analyzing practical flow situations. Through a detailed example of calculating the thrust of a jet engine, the lecture demonstrates how to apply these principles to solve real-world fluid mechanics problems. The choice of a suitable control volume is emphasized, as it significantly impacts the ease of calculations and the accuracy of results.