After learning Newton's 2nd Law, students in a university modeling-based introductory physics class are asked to imagine a box sliding across a floor and slowing to a stop. Although they've had extensive experience with friction in the context of energy, this is their first exposure to friction within the context of force. They are asked to make different representations for this scenario, including a system schema, and force diagram. During their small group work, students quickly run into a difficulty: there are only two interactions with the box (contact, gravitational), so there should only be two forces, yet the box is slowing, which means it must have unbalanced forces in the direction of acceleration. In this paper, preliminary evidence from a student-led whole-class discussion is presented showing how the group reasons through sharp disagreement in their initial ideas to come to a useful consensus.