This study focuses on the role of teachers pedagogical content knowledge (PCK) in transforming subject matter in physics instruction (at lower-secondary school). The purpose of the present study is to investigate the nature of teachers pedagogical content knowledge. Video recordings were analysed of 27 lessons taught by 8 lower-secondary school teachers on the topic composition of forces. A typology of content representations was developed inductively. The findings show that teachers use various types of content representations. These illustrate the dynamic nature of teachers pedagogical content knowledge, which is documented using the example of a teacher who combines various types of representation flexibly when introducing the concept composition of forces.