Students' challenges in learning mechanics are well documented from test situations, and group discussions are considered a fruitful way to meet these challenges. In this paper, we present a study from an authentic teaching setting where upper secondary students in groups solve the task of calculating the acceleration of an elevator by means of a scale. The group work was audio recorded in three groups with different ability levels. Analysis was performed inductively using the analytical tool of semantic gravity and semantic waves. The results reveal multiple challenges solving the task, even among high-achieving secondary physics students. The study shows that for group discussions to be fruitful, students must be able to negotiate for meaning by alternating between different levels of semantic gravity. In this study, only the group consisting of relatively high-achieving students was able to do this. For the groups that did not succeed, this is found to be due to insufficient knowledge base, poor integration of the required concepts in their own language and inappropriate epistemological framing of the situation. It is concluded that more effort should be put into learning basic concepts than curricula and teaching traditions normally provide. This should include tasks carefully adapted to students' ability, where students can practice alternation between levels of semantic gravity.