Phantom Forces Haunting Free Body Diagrams: Unveiling a Major Misconception in Statics and Dynamics

A concept found that students struggle with in statics and dynamics is free body diagrams. To capture the difficulties students have with this concept, faculty interviews were conducted to determine common errors seen in a classroom setting. The feedback was used to pull questions from a statics concept inventory focused on free body diagrams. Students who have passed statics and dynamics courses in their engineering degree were asked to participate in talk alouds to confirm the faculty input. The talk alouds provided first hand observations of the student thought process when they perform common errors in creating free body diagrams. The results highlighted that students commonly add forces that are not there or fail to only depict the external forces acting on the system in question. A professor describes the occurrence when students insist on adding forces that are not there as phantom forces. To remedy the idea of phantom forces, an intervention was proposed to be implemented into the classroom.