Developing a graphical tool for students to understand air resistance and free fall: when heavier objects do fall faster

Abstract

Many students find it difficult to apply certain physics concepts to their daily lives. This is especially true when they perceive a principle taught in physics class as being in conflict with their experience. An important instance of this occurs when students are instructed to ignore the effect of air resistance when solving kinematics problems. To a student, this assumption disconnects from their everyday experience. Mathematically, ignoring the effect of air resistance is crucial, however, since it renders such problems tractable. However, this step is rarely, if ever, provided with justification in undergraduate texts, leading students to believe that what they are taught does not apply to their everyday experience. Taking the additional step of clarifying when it is reasonable to ignore air resistance makes students’ reconciliation of their everyday experiences with the physics principle of free fall more likely. In this paper we develop a graphical tool intended to make this step as simple and effective as possible. We do this by summarising the results of a set of numerical simulations of various balls falling under the effects of both gravity and air resistance by means of a carefully chosen graph: a plot of an object’s cross-sectional surface are versus its mass. We further use our numerical results to evaluate how these two variables relate to the effects of air resistance for balls dropped from varying heights.