Abstract
This study explores the tension between subject knowledge and pedagogic content knowledge in primary teacher education. It documents students and in-service teachers learning about forces within the context of floating and sinking. In doing so it describes not only significant features of the learning process itself but also examines subject specific aspects of learning, identifying some of the inherent difficulties for learners within this domain and demonstrating how learners construct links between tacit knowledge and abstract scientific notions. Implications for teacher education and the teaching of science in the classroom are explored.