Although weight and mass are considered fundamental concepts in physics, they are still not well understood by students. A computer simulation was designed to improve students' learning of these concepts and compared it with other teaching strategies. The research was carried out with 142 students (7th grade; 12-13 years old), from three schools. There is a significant change in conceptual understanding of the concepts weight and mass for all groups. Nevertheless, total gains were higher for students who used the computer simulation. The implication is that using a computer simulation, carefully designed to address specific conceptual difficulties, may help the students understand the concepts of weight and mass. We also interviewed teachers to understand their role in the classroom. It was found that the features most likely to contribute to improve students' learning are related to the balance between support and autonomy given to students during the use of the computer simulation.