A mesoscopic approach to teaching fluid statics. Study of students' conceptions, design and experimentation of a teaching sequence. (In French)
This thesis presents: a study of the forms of reasoning learners apply when dealing with fluids; and the design, experimentation and evaluation of a teaching sequence on the statics of fluids in the presence of gravity. Moreover, it contains: an analysis of previous research on this topic by other authors; and some guiding ideas on the reasoning involved in this context. Content analysis and the study of conceptions are linked throughout.
The research on reasoning involves some interviews and several questionnaires, concerning the pressure in fluids and the Archimedes' thrust, addressed to high school pupils and first-year university students. The objectives are to study: what students think about pressure in fluids and how these conceptions change with educational level; how students imagine liquids and gases in their mechanical behaviour (ther¬mal effects are not considered), and how they reason about it; how students organize and coordinate different concepts and situations concerning fluid statics (pressure and force, atmospheric and water pressure, pressure and buoyant force, pressure and weight); and what difficulties and tendencies of reasoning, as more general cognitive modalities, students reveal about this topic.
One crucial point is a difficulty in connecting local actions and global effects. To promote understanding, it is not enough to say what a given quantity has to be; it is also necessary to explore how that can arrive, which modifications intervene so that things are as they are. This requires a systemic form of reasoning, based on the interactions between the parts of the fluid and on a transmission of changes that can explain the mechanism that makes it possible to establish the final situation of equilibrium.
Another significant aspect is the tendency of common reasoning to make confusion between contingent causes and efficient causes, i.e., between the conditions of occur¬rence of a phenomenon, what triggers an event, and the cause actually producing it.
The short teaching sequence experimented among first-year university students is based on a simple mechanical model of fluids, at the mesoscopic level, using sponge balls as elements. The aim is to make concrete and visual the typical decomposition used in fluid mechanics, using objects through which a behavioural analogy can be established. Certain students' conceptions are used as anchoring conceptions, to help bring about a conceptual change and to build a more complex and articulated reasoning structure. Attention is drawn to local interactions and causal reasoning, to help students overcome the difficulty of conciliating certain formal laws and the local behaviour of fluids. The mesoscopic character of the model is essential: a static model has to be mesoscopic, so as to hide within the unit of the model the kinetic aspects, whose effects are manifested by elastic resistance and pressure forces. To describe the effects of weight statically, the units of the model must be in contact with one another.
The sequence is evaluated by means of paper-and-pencil questionnaires as pre-tests and post-tests, whose results are compared against those of "classically" taught control groups, and by studying the transcripts of the student debates that were held in the final session. The results show that the implementation of the mesoscopic model proved effective as regards the development of systemic reasoning and the reconciliation of local and global approaches.
The observations of a group of trainee-teachers are given, regarding the implementation of the sequence in class.
Some directions for developing the research are suggested at the end of the thesis.
Supervisor: professor Laurence Viennot
For further reading:
L. D. S. P. Laboratoire de Didactique des Sciences Physiques
University of Paris "Denis Diderot" (Paris 7)
2, place Jussieu
75251 Paris Cedex 05 - France