Seeing is (not) believing
I’ve seen research cited that suggests demonstrations are not always effective because students often remember what they think they are going to see, rather than what they actually see. That is, prior conceptions can overwhelm actual observation. Thus, it is better to have students actually touch the apparatus, take the data and “see for themselves.” That’s what I do by using Modeling Instruction in Physics. But just taking the data isn’t enough, as was demonstrated today in my class.
My class had produced system schema and free body diagrams for a series of situations involving two carts in contact. These situations included constant and non-constant velocity, equal mass carts and unequal mass carts and even a collision between carts of unequal mass. While the students tried to puzzle out the FBD’s together, great questions about the relative magnitudes of the forces on the two carts come up. We then moved into the lab where the students were set up force sensors mounted on carts so they can try out each of the situations for which they’ve been drawing FBD’s. After data was taken, we went back to the classroom and each lab group whiteboarded one of the situations and explained what they found regarding the force that each cart exerts on the other cart.
The first group that reported on a non-constant velocity situation reported that the force that cart A exerted on cart B was, in fact, larger than the force that cart B exerted on cart A. The class sat silent while this sunk in. I asked if everyone’s observations coincided with this group’s observations. Total silence. After a bit of prodding to take a stand, they all admitted that they had measured the two forces to be equal. At that point, the group presenting admitted that their own data showed equal magnitude forces, but they just thought they should be unequal, and “kinda remembered” that they were unequal.
This led to a great discussion about our ability to fool ourselves, and how science can be thought of a set of behaviors designed to avoid fooling ourselves. The important point, I think, is that only by allowing students to present and defend their results (making mistakes and owning up to the mistakes in the process) does such an opportunity come up. Today’s class felt like a huge win, and I sincerely hope my other section of Honors Physics makes some kind of mistake just this good tomorrow.
Physics&Parsimony 
That’s a great moment, Mark. As encounter more of these situations, both in my own classroom and through others sharing, I pass them on to colleagues. They seem skeptical at first as if students really wouldn’t be able to fool themselves at the levels we see. I guess I’m starting to wonder – is this phenomena only found in science classes, or even more strangely, only in physics classes? Is it because students have such entrenched models of the way the world works already that their resistance to changing them is so high? Could it be that in another field, say history or math, they don’t encounter this because students have never considered exponential functions or the causes of the Great Depression?
Brian
2011/09/27 at 22:35