This paper deals with the joint construction of knowledge by the teacher and the students in a physics classroom. It is focused on the status of epistemic certainty/uncertainty of knowledge. The same element of knowledge can be introduced as possible and thus uncertain and then evolve towards a status of epistemic certainty; the status of other elements can do the reverse. The evolution of a certainty/uncertainty status reflects the evolution of the shared knowledge in the classroom. The study of this evolution is based on a previous analysis of the evolution of knowledge in a classroom during a teaching sequence of mechanics at grade 10. From this analysis two notions were selected and the evolution of the elements of knowledge associated was analyzed in terms of epistemic certainty/uncertainty. The results show how the emergence of new epistemic questions depends on the nature and status of student's prior knowledge; in other terms, new epistemic uncertainty emerges from epistemic certainty.