Probing the influence of the teaching method on students' ability to identify real forces in diagrams

When solving physics problems, students often fail to identify real forces acting on the body under consideration [1]. This problem is mainly due to the abstract nature of the force concept, but it can also result from the traditional teaching methods. Typically, the mathematical approach to physics problem solving is characterized by introducing additional, nonexistent forces, thus leading to a loss of physical clarity. Vector calculus, in most physics courses, is carried out by the traditional algebraic methods, i.e. by resolving the forces into components and then summing up the components of the same direction. In this procedure the number of the vectors appearing in the force diagram is significantly increased. Although this procedure facilitates the calculation, it potentially leads to the misconception that the components are also some real forces. In addition, the procedure is not entirely in accord with the concepts of the Newton’s laws which state that the motion of the body is determined by the vector sum of the forces, usually not considering the components. The question arises: is the direct vector method [2] or application of the polygon rule for vector addition more successful in teaching mechanics? In this study, we have developed an instrument that aims to measure the extent to which students can identify the real forces in different diagrams. Unlike the FCI test, which mainly checks for preconceived notions, our test checks for didactogenic misconceptions. In addition, our multiple choice questionnaire is based on the visual representation, where the students have to choose the diagram that correctly presents only the real forces. Using this instrument we made an initial study of the effectiveness of the two different problem solving methods. Our test, consisting of 12 items, was administered to the two groups of first-year physics students at Rijeka (RG) (n = 27) and Split (SG) (n = 25) University, after the “concept of force” had been covered in class. We found the pretest item difficulty index ranging from 0.12 to 0.83, with the discrimination values from 0.21 to 0.93 and the KR-20 reliability estimate 0.72. Afterwards, both groups exercised additional force diagrams tasks for a period of two class hours. The only difference was that RG used the polygon rule and SG the vector components method. Posttest showed a shift in the average rate of correct responses which was larger for RG. The normalized average gain in RG was 0.46 compared with 0.24 in the SG. The ANCOVA showed a statistically significant difference (p=0.009) in favor of RG. Based on these initial results we formulate the working hypothesis for the future investigation: If we apply the teaching method where the force diagrams are solved, not by separating force vectors into the components, but by adding them by using the polygon rule, we foster students' ability to identify real forces and we improve their understanding of Newton's laws. Further tests of this hypothesis are the subject of our future work. References [1] For a recent review of literature see e.g. A. Savinainen et al., Phys. Rev. ST Phys. Educ. Res. 9, 010104 (2013). [2] A.S. Kondratyev, W. Sperrym, Phys. Teach. 32, 417 (1994).