There was a recent discussion on Twitter about using the term “preconception” instead of “misconception” when talking about student difficulties. The debate goes back many years, but it’s still worth reviewing. So: How should teachers deal with the problems students have in introductory courses, and what should we call them?
Let’s start with a particularly common student idea. Suppose I get a low-friction cart and give it a push so that it moves along a track like this.
Casual observation seems to suggest this cart is moving along with a constant speed—and that’s essentially true. But why? Why does the cart move with a constant speed after it is pushed? Here, I will make this a multiple-choice question for you.
A. The cart moves at a constant speed because there is a constant force pushing.
B. The cart moves at a constant speed because the original force from the push is transferred to the cart.
C. The cart moves because there is no force acting on it.
D. The cart moves because of some other reason (and describe the reason).
I’m pretty sure many students would agree with one of these options. In fact, the most common response will probably be a combination of A and B—that the cart moves because there is a constant force pushing it. If you ask them to name that force, they might call it “the force of motion” or “the force from the push.” Of course the answer that is supported by physics models is that the cart moves at a constant speed because there is zero net force acting on it.
Now, do students have a misconception about this situation? Are they just plain wrong? Nope. Here is the important thing to remember: Students have ideas that are based on something that makes sense. Humans don’t just make up crazy stuff. (Not normally.) Instead we build ideas based on previous experience and these ideas have to have some sort of logic.
How about a similar case? Here is a block without wheels. How can we get this to move at a constant speed? Yes, you need to pull it. This is what it would look like.
This is something that pretty much everyone has experienced. Maybe it’s pushing a book across a table or pulling a chair across the floor. It’s clear that if you want to move it at a constant speed, you need to push with a constant force. This idea of “constant force means constant motion” should apply to the low-friction cart too. Of course, there is one very significant difference between the two objects that move at a constant velocity. The second block had a frictional force acting on it, whereas the rolling cart had essentially no friction. You can’t really see the friction and it’s sometimes difficult to see that it is a force, but it’s there. So in both cases the object moves at a constant speed with zero net force.
Hence, the “misconception” here isn’t 100 percent wrong. By calling it a misconception, we are telling the students that they are flat-out wrong. But their ideas aren’t bad, just developing. That’s where the “preconception” term comes from.
Who cares about this stuff anyway? Well, it is important. If you want to structure a course to help students develop and build ideas, you have to know where they’re starting. The instructor really needs to know what these initial ideas are so that you can present the students with new situations that help them modify these ideas.
There are two ways you can figure out what your students think about some particular concept. The most obvious way is to just ask them. Really, it’s a good idea to do a little check every once in a while. Give the students some activity or question that deals with the topic. For the forces example above, just ask what forces are on the cart after it was pushed.
The second method to determine students’ preconceptions is through experience. For the forces on a cart question above, I didn’t actually ask any students. However, I’ve been teaching those ideas for quite some time. I have a pretty good feeling for what they are going to say (although students can still find a way to surprise me once in a while). If you don’t have experience with students, many textbooks have materials for instructors that describe some of these common ideas. There is also the book Teaching Introductory Physics by Arnold Arons (Wiley, 1996). This book goes over a wide range of topics in introductory physics. It’s a classic.
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