This paper deals with the way that students interpret relationships between physical quantities with respect to time, especially those involving physical quantities that are considered at the same time. Research findings show some important difficulties which are probably linked with a causal interpretation of phenomena. The main features of “linear causal reasoning” about systems, as opposed to a quasi-static analysis, are described and illustrated. The paper ends with a discussion about pedagogical goals.Cet article concerne la manière dont les étudiants comprennent les relations entre grandeurs physiques du point de vue du temps, en particulier celles qui concernent des grandeurs considérées au même instant. Des résultats d’enquêtes mont...