Investigation of student reasoning about air resistance and terminal speed behavior of falling objects

Introductory physics courses often present simple models in which salient, real-world factors are ignored. While sensible given constraints of time and student background, this approach can be unsatisfying for students and instructors. A velocity-dependent air resistance force, when incorporated with free-fall due to Earth's gravity alone, provides an accessible yet realistic model that can account for terminal speed behavior. Applying this model requires students to coordinate the drag force with the zero net force condition for objects moving uniformly. We have developed written questions and interview tasks to investigate student understanding of these ideas in the context of objects falling at terminal speed. In this talk, we present student responses as evidence of specific difficulties. We find that many students focus on a single variable, such as the cross-sectional area of the falling object, and give responses that seem to contradict previously learned ideas about force balance. The talk will include interpretation of selected reasoning difficulties using dual-process theories of reasoning drawn from cognitive psychology.