This thesis is further based on a literature study, in which I took part, where research on difficulties in understanding Newton's third law was compared. In order to investigate how physics teaching on this subject can be designed with a variation theory perspective, an intervention study inspired by a learning study was conducted. The study's object of learning was to develop the student's ability to describe how action and reaction change the object's motion in everyday situations. To fulfill the purpose of the work, the research questions were: What knowledge and understanding about force and motion did the students show pre- and post-teaching? What do the students need to discern in order to understand the object of learning of the intervention study? What patterns of variation enable the discernment of the learning object's critical aspects? The sample consisted of 41 students in year 6, divided into three classes. The data collection consisted of pre- and post-testing and analysis from three lessons. The critical aspects that emerged in the result were: discern that motion is explained by an imbalance of forces, discern that forces arise in interaction between two objects, discern that one of the objects can be the ground (for example a road), discern equality between the objects, human and "non-living”, in force situations and discern that friction is a type of force. Patterns of variation used were contrast, separation and generalization. The comparison between pre- and post-tests revealed that the students developed their knowledge and understanding of the study’s object of learning.