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.

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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.

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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.

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