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.