The general topic of this contribution is the knowledge of science teachers, involved in their lesson preparations and implementation in class. We investigate the specific knowledge that the teacher builds for the teaching of particular topics in sciences. Firstly, we try to identify which kinds of knowledge are used by the teacher to make the subject accessible for students (Geddis et al. 1993; Grossman, 1990; Shulman, 1986, 1987) through preparation of classroom; then we try to follow the evolution of knowledge during the classroom practice, by observing in particular the gap between what was planned and what has been achieved. The evolution of teacher’s knowledge during classroom has an impact on the result of his teaching. Our research focuses on a sequence of about seven weeks, in order to take into account long-term processes. The study of the different kinds of knowledge mobilized in the preparation of the classroom refers to the concept of pedagogical content knowledge, the work of Shulman (1986, 1987) and to the expanded model of Magnusson and al. (1999), taking into account the experimental dimension of this teaching. Moreover, in our didactic study we draw on the work of Martinand (1986) about the plan of the modelling and the conceptualization from the constitution of an empirical referent, and the work of Coquidé (1998) on three didactic modes which obey the experimental practices.

We study the case of two teachers of physics and chemistry at middle school (8^th grade) in teaching mechanics. This course, and more generally the teaching of science and technology, should enable the student to pursue in particular a scientific approach, to manipulate and to experiment, to express and exploit the results. In this context, we chose to study the preparation of the teacher and its classroom implementation.  During the preparation, we look at the knowledge bases (Shulman, 1987; Grossman, 1990) that the teacher transformed to provide the students with good conditions for learning. This knowledge is going to allow him to define orientations during the sequence or for one lesson in particular regarding the experimental practices and the modelling. The choices of the teacher are recorded in a log book throughout the studied sequence. They are analyzed and commented during the interviews which take place before the observed session. During the act of teaching, the teacher has to adapt himself and adapt his teaching to the particular situation, that he did not necessarily plan in his preparation. We observe these situations, and we try to identify, by drawing on the sequences videos, the knowledge involved and the impact on the rest of the course and sequence.

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