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Focuses on difficulties linked to situations in physics involving two models--geometrical optics and wave optics. Presents content analysis underlining two important features required for addressing such situations: (1) awareness of the status of the drawings; and (2) the 'backward selection' of paths of light. (Contains 24 references.) (Author/YDS)

When a fluid fills up a rotating box, the pressure inside the fluid is easily obtained by solving a closed set of equations. However, if the fluid is incompressible, the mass conservation equation is trivially satisfied and the pressure remains undetermined. After a preliminary analysis of the similar problem of a rotating rigid rod, this paradox is removed by giving the fluid a small value of the compressibility [Chi] and taking [Chi] [approaches] 0 at the end of the calculation.

In this article an analysis of the difficulties experienced by students when applying the principle of superposition to electric fields is presented. A study of university level students in France and Sweden revealed the existence of difficulties arising out of two issues: (1) a causal interpretation of some relationships, (2) the student's need for an effect, motion of some kind, to accept the existence of a field. The links between these obstacles and a lack of a unified view on electric phenomena are discussed from a pedagogical point of view.

This article is focused on elementary algebra in physics. After a brief recall of what should be understood in this topic, the paper examines common difficulties. Results of inquiries conducted with students at university level (first to fourth year) and with trainee teachers are presented and discussed. The authors point to the advantages of using schemes that explicitly specify two different kinds of indications: coding arrows, on the one hand, and realistic arrows on the other. Some examples of questions suitable for an interactive work with students are proposed.