In this clip, the Professor is addressing students at their desks. The Professor explains that most of the groups have made motion maps, but he decides to split up the class and assigns each of them a different direction of acceleration. The students then determine that since there is acceleration, the velocity has to be changing as well. Therefore, they conclude, every time you hit the ball it accelerates.
This clip takes place in week 13 (Investigating Spring Forces & Circular Motion). The problem that the students are working on is number 1 of the “Collision Problems” worksheet.