T. McCarthy, T. Goldfinch

University of Wollongong (AUSTRALIA)
There are many threshold concepts for students to grasp in Engineering Mechanics. Research has shown that one of the critical concepts that must be understood is the Free body diagram (FBD). Students who do not fully understand this threshold concept are shown to perform badly in subjects such as Engineering Mechanics which itself is a foundation subject in most engineering degree programs.
Free body diagrams are the abstraction of the external forces and moments acting on a physical object. They simplify the force system. Some Engineering students “get” FBD’s the very first time they are shown one. Many struggle and continue to produce incorrect diagrams well into their university studies.
The nature of a threshold concept is that when mastered, it transforms the student’s understanding. Like riding a bicycle, once understood it is hard for forget how to do it. Finally, not understanding the concept is a complete barrier to mastery of content that relies upon in.
It was reported in many papers and also by teachers of Junior (3rd year) and Senior (4th year) subjects that FBD skills were still below expectations. An intervention was implemented in a University of Wollongong first year subject to (a) test students’ understanding (in Bloom’s taxonomy) of FBD’s (b) diagnose common misconceptions and errors and (c) reinforce the importance of constructing FBDs that are accurate and complete. This intervention has taken the form of a staged mastery skills online test early in the first session of the first year of the engineering degree courses. The online test is used as a stage gate for the subject and everyone must achieve the mastery skill to pass the subject. Multiple attempts are allowed.
This paper describes some design of FBD questions and how they are used to discriminate between those who have mastered the concept and those who have not. Some FBD misconceptions and mistakes are outlined with strategies for teaching the correct approach. Finally some correlations are given between performance in the skills test and subsequent performance in engineering mechanics subjects and design subjects. The test is seen as a good predictor of future performance in higher level mechanics subjects.