Passive and Active Forces
When students are asked to identify the forces acting on a book sitting on a table, some fraction of students will say it's only gravity pulling the book down. The table, they might argue, doesn't push up on the book. The table simply gets in the way–it blocks the book from falling.
One way of making sense of this is that students think of forces as pushes and pulls. In order for something to push or pull, then, it has to be active in some way. For example, you expect to see a person straining their muscles to push or pull. You expect to see a spring being compressed or stretched when it pushes or pulls. The table, students might think, can't actively push or pull, so it can't be exerting a force.
Here students seem to have the wrong idea, so we might tempted to think "forces as active" as a misconception, causing students to think that the table doesn't exert forces. From that view, we'd want to change the students' conception of force, so that they would understand why a table does exert a force. Here are four reasons not to think this.
Reasons #1 Not to think of this as a misconception
Wrong answers are not necessarily indicators of bad ideas. John Clement, for example, saw in students' notion of active force a quite productive idea. So instead of trying to change students' conception of force, John set out to help students to "see" the table as being alive and active. His instructional strategy aimed to help student see the table as a "springy" surface that just happens to be very stiff. In this case, it wasn't the conception of force that needed refining, it was the conception of table.
Reason #2 Not to think of this as a misconception
Ways of thinking you don't like now are often the ways of thinking you'll want back later. From an introductory physics perspective, we want students to think about surfaces exerting normal forces. So if they don't think of tables as exerting forces, that's a problem. But the truth is, later we'll want physics students to think of surfaces as merely constraints upon motion. So if students think of surfaces as blocking motion, that's (kind of) exactly how we want them thinking about it. So, what was once a misconception has suddenly become sophisticated.
Reason #3 Not to think of this as a misconception
Force is not a singular concept, and neither are students conceptions of it singular. Force is more of an explanatory framework. There are many bits and pieces that have to come together. The only reason it makes sense to think of the table as exerting a force on the book is once you've put all the pieces in place.
To put this more clearly, thinking of the table has exerting a force has a lot to do with having a commitment to the notion of equilibrium and a commitment to the notion of net force as a explanation for equilibrium. So, what seems like a question about force is really a question that about a person's level of commitment to a whole framework. Included in that framework is ideas that interconnect the ideas of force, net force, and equilibrium.
Reason #4 Not to think of this as a misconception
Concepts and language are not the same. If you don't mention anything about force, and ask, "What's holding the book up?", every child and student will say the table is holding the book up. So even though they don't think that the word "force" should describe what the table does, students DO think that the table is interacting with the book. From this perspective, students have the right idea, they just don't think the word force should apply.
The Big Idea
I think my point is this. It's fine to think about misconceptions, as long as it doesn't stop you from doing the kinds of thing I just did.
What did I do?
- First, with the help of John Clement, I spent time thinking about the potential productivity of students' prior knowledge and how I might capitalize on that for classroom learning.
- Second, I tried to see connections between what students know now, and what kinds of knowing I might expect them to know soon (and also down the line).
- Third, I thought about all of the ideas that students would need to have in place, and resisted the temptation to think of the problem as simple application of isolated knowledge
- Fourth, I spent time thinking about what the ideas students have, and resisted the temptation to evaluate students' ideas based on vocabulary alone.
test comment
ReplyDeleteBrian,
ReplyDeleteThis is another great post that gets me thinking about how to really build on student's knowledge to really help them construct their own understanding and not simply substituting it with my own. I must confess that often after our first forces lab, I "guide" my students to build a careful template to describe forces like the following the [type of force] of the [object exerting the force] on the [object experiencing the force], and we practice it over and over to get rid of notions like non objects like gravity exerting forces and instead think of the earth exerting forces. I do this because it pays huge dividends when it comes to N3, but I wonder if there is a better way to get them to develop these ideas without so much structure from me.
Also, it occurs to me that most of this top-down, vocabulary driven approach to science seems to get engrained into their heads from a younger age—probably in middle school, which is filled with vocabulary to memorize, procedures to learn by route, and 5 step scientific methods to follow. While there are curriculua out there like IPS that avoid this, I think in general, these curriculua place such demands on the expertise of the teacher that many middle school science teachers just stick with the more familiar, learn it this way approach. What are your thoughts about how to change that?
So this is a bit of a rambling response to just the first part of your comment, but here goes:
ReplyDeleteI have no problem with teaching conventions. In math, for example, it's important for students to learn order of operations. But students shouldn't be led to believe that order of operations represents mathematical thinking, when it's just a convenient choice we made a long time ago. Other conventions are ones we make local in our classrooms, like how to label forces.
So the question for me is, "Why do students think they are labeling forces this way? What problem does it help address for students?" If the only reason we can give students is, "Trust me. In the next unit on Newton's 3rd Law, it'll pay dividends," or "I need to drill out of your mind the idea that gravity is a non-object force", then I'm not entirely satisfied. That being said, we can't ignore that it DOES seem to pay dividends. So the question is, can we do better?
I think we can. For me, it's a matter of viewing learning this stuff as genuine puzzles to be sorted out. Saying, huh, you know I can identify that it's my finger pushing the block, and you know how I can identify that it's the table holding up the book. So what's doing the pulling with gravity? Isn't that weird that gravity seems different than other kinds of forces?
It helps me when I view the issue as an honest intellectual puzzle, because it reminds me to treat it as such. My job is to help my students make contact with that puzzle, and to be patient with them as they maneuver through it with my guidance. And part of my job is also to get them invested in the puzzles.
And resolving the gravity puzzle is certainly not going to occur simply by having a better system for labeling forces, despite the fact a better system might be a good way to keep track of important stuff. In fact, the resolution is one that progresses and develops over years and years, as one learns about the universal law of gravitation, and how weight is the net force due to many Avagadro's number of atoms tugging together, and then learning about gravitational fields as a way of not having forces acting at a distance, and then perhaps learning about general relativity or gravitons. Of course you aren't going to teach students all of this, but my job as a teacher is to nudge them along.
I suppose I'm not so much concerned with structure, per se. It's a matter of what we are trying to structure.
It would be an interesting question to ask your students why think they your class labels forces this way. And if they think scientists label their forces that way. No matter what they say, I think it would be interesting.
Brian, thanks so much for articulating this so clearly. I think I do many of the things you describe above as we are setting out the way we label forces, but I don't think I do it nearly as thoughtfully as you describe. This is a great challenge for me to keep working on.
ReplyDelete