This presentation aims at describing how an experienced teacher copes with the transition from students' viewpoints to an authoritative standpoint when teaching the principle of inertia in a French secondary school (grade 10). In order to describe the teacher's actions, this case study is based on the joint action theory in didactics (Sensevy, 2012) and on the analysis of classroom video data. The results show that the students took the floor to defend their opinion about the mechanical actions exerted on a moving ice-skater. They also point out the teacher's difficulties in having the students accept the teacher's proposals leading to the principle of inertia: the transition from the students' viewpoints to the teacher's authoritative standpoint, made under strong teacher's incitements, is either abrupt or based on ambiguous arguments. This can be related to the nature of the given starting situation that mixes reality and modelling levels without the teacher (and the students) being aware of it, and therefore to the teacher's viewpoint on the nature of science.
