We talked about how which forces are internal and which are external change when you change the system boundary. We saw how the many forces of gravity become just one when we make the system larger. We defined the mass of the system and the position of the system (the center of mass). And then we tried our model on two air pucks that were previously filmed. We saw no obvious external forces, but the position-time graph wasn't a straight line; the velocity-time graph was. Turns out the system was on a hill, and we need to explore a new type of motion.