This study aims to better understand the construction of the meaning of physics concepts in mechanics during a teaching sequence at the upper secondary school level. In the teaching sessions, students were introduced to the concepts of interaction and force. During this teaching sequence the models called "interactions" and "laws of mechanics" are successively introduced in the framework of tasks involving a variety of material situations. The hypothesis "students' initial knowledge on the verb "to act" is a founder notion, to the extent that its meaning plays a crucial role in constructing the concept of interaction" has been set. The research questions are: a) Does the verb "to act" plays a founder role in the construction of the concept of interaction? b) What are the other notions that intervene in the structured set of knowledge that students use to construct the concept of interaction in the teaching sequence? The results of the study show that, for the two students of the observed dyad, the notions of object and the concept of gravitation are simultaneously founder notions. Additionally, the two students of the same dyad who work together all along the sequence use different categories of knowledge and construct different meanings of the concepts.