After going over homework on force pairs and forces at angles, I divided the class into six groups and had each group draw an interaction diagram and a free-body diagram using the situation pictured above. Here's what they came up with:

The interaction diagrams looked different because the objects for the FBDs were different. Each group had a different system of interest, so each group saw the situation differently. This insight allowed us to quickly define internal, external, and negligible forces.

Then we investigated what point should we track on video analysis to see CVPM for various systems. I'm working with some great friends and colleagues I met at the Ohio Modeling Workshops in Columbus, Ohio, and we're working on a manuscript for the Physics Teacher, so I don't want to go into too much detail here. (I'm already thinking of little tweaks for next time.) But my colleagues and I are trying to develop systems thinking in physics, and so far, so good. My first try with students worked well today.