The issue of students' engagement in learning physics has generally been analysed in science education research using attitudes towards science, or motivation to learn science. However, the corresponding studies have their limitations. Therefore, the object of this paper is to present and to analyse an alternative to these theoretical approaches; namely the "relation to knowledge", which has been used more and more in the past 10 years in French-speaking educational research. The description and discussion of the broad outlines of this theory, of the associated methodology, and of the obtained results regarding the learning of physics will show the significance of this approach and its limits, as well as the perspectives that it opens. (Contains 15 notes and 1 table.)