We finished talking about the central force model by summarizing how we could apply kinematics, forces, energy and momentum to circular motion. Talking about momentum with the central force model was new to me this year, but, if you think about the planet-star system as having no non-negligible external forces, then the center of mass stays at rest or at constant velocity. When the planet goes one way, the star goes the other way. Such an obvious way to see them both rotating around the center of mass of the system.
Then we drew force diagrams for this situation:
We weren't sure how to draw the tension forces; don't they cross each other? And how do we show that the little mass is hanging down on the meterstick in the middle of the meterstick? When we moved the little mass, the whole setup was ruined.
We broke the particle model. We need to model this meterstick as something other than a dot. Off to lab to figure out how to keep something balanced when we care about where the force is being applied.