In this paper, we analyse learning dynamics in the context of physics group work of the kind increasingly found in engineering education. We apply a phenomenographic perspective on learning, seeing the notion of variation as the basic mechanism of learning. Empirically, we base our analysis on data from first year engineering students discussing physics problems concerning force and friction while working in small groups of three or four. The discussions were captured on video and audio, and the subsequent analysis primarily relies on detailed transcriptions and the students' notes. The results illustrate how students relate different parts of the whole learning object (Newtonian mechanics) to one another and create a variation with respect to the parts and/or the whole; how the presence and experience of variation complemented by an experience of relevance may result in identifiable learning; and how tutor interventions may have a favourable impact on the learning dynamics. (Contains 1 figure.)