Engineering students in control courses have been observed to lack an understanding of equilibrium and stability, both of which are crucial concepts in this discipline. The introduction of these concepts is generally based on the study of \classical" examples from Newtonian mechanics supplemented with a control system. Equilibrium and stability are approached in di erent ways at the various stages of a typical engineering syllabus: at the beginning, they are mostly dealt with from a static point of view, for example in mechanics, and are subsequently handled through dynamic analysis in control courses. In general, there is little clari cation of the di erences between these concepts or the ways in which they are linked. We believe that this leads to much confusion and incomprehension among engineering students. Several studies have shown that students encounter di culties when presented with simple familiar or academic static equilibrium cases in mechanics. Our study investigates students' conceptions and misconceptions about equilibrium and stability through a series of questions about several innovative non-static situations. It reveals that the understanding of these notions is shaken when the systems being studied are placed in inertial or non-inertial moving reference frames. The students in our study were particularly uncertain about the existence of unstable equilibrium positions and had di culty in differentiating between the two concepts. The results suggest that students use a velocity-based approach to explain such situations. A poor grasp of the above fundamental concepts may result from previous learning experiences. More speci cally, certain di culties seem to be directly linked to a lack of understanding of these concepts, while others are related to misconceptions arising from everyday experiences and the inappropriate use of physical examples in primary school.
