Which of the following is not considered an appropriate classroom demonstration for explaining the four basic forces of flight?

In the context of teaching the four basic forces of flight—lift, weight, thrust, and drag—it's important for demonstrations to accurately represent these concepts within their relevant context. A weight on a string is commonly used to demonstrate centripetal force, which pertains to circular motion rather than directly illustrating any of the four basic forces of flight. Therefore, while it is a valid physical concept in aerodynamics, it does not directly support an understanding of the fundamental forces acting on an aircraft during flight.

In contrast, using a cardboard model to demonstrate lift effectively illustrates how air pressure differences create lift. Employing a fan to demonstrate thrust provides a clear representation of how engines propel an aircraft forward. Similarly, using water to explain drag can effectively visualize how resistance acts against an object moving through a fluid. Each of these methods is closely tied to the respective concepts of flight forces, making them appropriate for classroom demonstrations of the four basic forces.