We started today by trying to figure out when constant velocity happens in the real world. We weren't sure, as a class, if we had two fans of unequal strength, if the cart sped up to a certain constant speed or was always speeding up. We had to take the cart off the track and put it on the floor to check.
Then I talked about the four major types of interactions (gravity, normal, friction, and tension) and about how to make an interaction diagram. Then we practiced making interaction diagrams and free-body diagrams.
My students don't know yet how important interaction diagrams are. Wait until they see how we use them in MTM and ETM.