In this clip, students are gathered around in a whiteboard circle. One student addresses the class with a question, asking if the box attached to a cart is moving or not, so they would know whether to use static or kinetic friction. The professor points out that the box is, in fact, moving horizontally. The class and the professor decide that since the box isn’t moving relative to the surface of the cart, then static friction should be used.
This clip is from week 12 (Practicing with Forces & Exam). The problem being discussed is problem number 2 from the “Possible Force Problems” activity worksheet.