# Days 11 and 12: Talia’s Law (N1L), System Schemas, and FBDs

After the bowling ball investigations, we returned to the classroom to look at the hover puck and the fan carts. We talked a little about confirmation bias and how difficult it is to really observe (rather than explain why you expect to see something). We spent a lot of time observing what happened when the dueling fan carts started at off-vs-high and then changed to high-vs-high. Did the carts slow down when the second fan turned on or did they just stop speeding up?

Eventually, one student suggested that we have the high-vs-high portion happen for longer—that helped us see that it really was a constant speed once both fans were turned on.

Talia was the first to clearly articulate the idea of “no net tapping” and how that was associated with a constant velocity, so she earned credit for our (and Newton’s) first law.

We started our common types of forces table (to get a common vocabulary), then built up the idea of system schemas and free body diagrams (FBDs if you want to text about them) using the fan cart situations as our guide.

An interesting part happened when we diagramed high-vs-low. I guess we didn’t spend enough time observing it the day before, so there was disagreement over whether it would be a constant velocity or a changing velocity. Some students pointed out that Talia’s Law would say that it must be a changing velocity because the forces were clearly unbalanced. We pulled out the track and carts again to check—it did speed up, and Talia’s Law was verified once again.

We ended by getting started on the big box-shoving problem that will take up our next few classes. I’m excited to see them battle out the first part in our next class. There are already a few different ideas popping up on different boards around the room.