In this study, a progressive-learning approach to physics, based on knowledge-building pedagogy, was compared to a content-centered approach in which explanations, experiments, and discussions are centered on the transmission of knowledge. Forty-six students attending the first year of high school participated in this study over a whole school year. Students' knowledge and mastery of physics concepts were assessed through questionnaires containing both open-ended and multiple-choice questions. Overall, the "progressive-learning" group outperformed the content-centered group. Results are discussed in relation to the theoretical background and the experimental teacher's diary of classroom activities. The main conclusion achieved by this study is that the teaching of physics should be slow, cyclic, and developmentally appropriate for the context.