Yesterday, in E1, Table Six was whiteboarding a story about forces. Lane and Adam act out the scenario for the classroom: Lane's sitting in a chair. Adam comes up and pushes the chair, but Lane and the chair do not move. Melanie draws not one, but two pictures with her group to illustrate the forces in this scenario.
We get a little slice of her rationale in this video clip. Adam comes over to check in with Melanie, who's alone at the table. Here's their conversation:
Adam: What you got? So this is. . .
Melanie: This is you pushing on the chair and friction pushing back and everything else staying the same.
Adam: So what don't you like about that one?
Melanie: I can't see it. That's a dot and some arrows. This is a chair, these are people.
Adam: So, can't you say that they're taking this chair and just drawing it as a dot so it's simple. Well it's look [incomprehensible]
Melanie: Yeah, I know, I did this, for those minded people, this one I need the chair. I can see the chair.
Adam: Yeah, well what's nice though, if you look-
Melanie: -it's exactly the same-
Adam: -it's the same. . .
Melanie: It's because it's not moving
Adam: So the fact that it's not moving is like a force
Melanie: If you're pushing harder, it will move
Adam: You have to overcome like this . . .
Melanie: . . . force . . . that's with you?
Melanie: They made me add it. . .
So, I'm interested in this clip because of how Melanie tells Adam that she can't "see" the dot and arrows. Instead, she says she needs the chair. She's made sense of this scenario by drawing the actual objects. To me, this seems like it could be an example of needing to work through the abstract by drawing on experiences with concrete interactions with actual things. Is this why she needs the chair and the people? Because, well, what can we know about how forces act on a dot from our real life experiences? I've never applied force to a dot. I've never sat in one or dragged it across a floor. We make use of our sensorimotor experiences with real life objects in order to work out abstract things like the forces acting on an object.
Also, I noticed that the arrows are drawn in both pictures so that they all point at the chair. The "Lane push on chair" is drawn with the arrow head on the chair and the "push of chair on floor" arrow is drawn with the arrow head on the chair. The other two forces have arrowheads on the chair, too.
It seems like maybe the orientation of the arrow heads indicates something about perspective, here: of the stuff doing stuff to the chair. Could this be an artifact of embodied thinking? If you were thinking about what's happening by "doing" the forces (embodying them) you might think of yourself doing the push and you'd orient the arrow in the direction you'd act in as the forcer. So, if I'm the ground I push up (arrow head points up to the chair) and if I'm the mass in the chair I push down (arrow head faces down to the chair) and if I'm the friction I push against the opposing motion of the chair, and if I'm the pusher I push against the friction (arrow heads meet on the chair). A closer look at Melanie's gestures in her explanation might support this.
After I got all excited about this, I realized that I have no idea how to draw a free body diagram and so I solicited help from Benedikt. Turns out, having the arrowheads facing the object is not how a typical free body diagram is drawn. After that, I had some other questions that I talked to Rachel and Benedikt about. They were:
Why are students asked to draw free body diagrams in the first place?
What is force? (How do you talk about force without making it a thing?)
I'm not sure I can really think about forces as not being things. Maybe it's because there aren't forces without things. In Melanie's pictures and free body diagrams forces are arrows. Arrows are things. Rachel talked about how force is the doing and that maybe force should be called force-ing. The "ing" makes it verbier.
I've also been noticing this week that there's a lot of gesturing that happens when people are talking about forces. You don't have the arrows when you're speaking. It seems like people do a lot of squeezing, pushing, and pulling in the air, maybe to makeup for that. Are they tracing paths? Feeling forces? Embodying the forces?
Going back to the episode, in the very beginning (13:26), Melanie seems to be using her hands to represent all the different forces in her picture. For example, in this picture she says "and friction pushing back." She's using both hands to poke the invisible chair from both directions, showing, but not saying equal opposing forces.I'm going to keep exploring the role of gestures when people are using their hands while they're talking about forces in a future post.
In the mean time, maybe I can blame Yoda for why I think of force as a thing: The force is. . . with you. Like, in your pocket?
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