The purpose of this study was to investigate the contribution of a computer simulation to students’ learning of physics concepts (weight and mass). Our simulation was produced using the software Modellus. This study evaluates progresses in understanding made by students (grade 7; 12-13 years old) after one lesson (90minutes) in three different scenarios: using only “hands-on” activities, using only a computer simulation, and using both. The progresses were measured through pre- and post-tests. The results show that the total gains were higher when students used the computer simulation, alone or together with “hands-on” activities. However, we found that the total gains obtained depend on the teachers pedagogy when using the computer simulation to teach the concepts of weight and mass