# We Have Met the Enemy and He Is Us

In a comment to the AP post, “hogeb” asks an excellent question about pedagogy:

I’d like to enlist your advise and the advise of any readers who can provide it. I teach physical science to pre-service elementary school teachers. I try to elucidate the somewhat subtle differences between the application of a force and the just getting in the way of, among other things, and I try to point out why this isn’t just semantics but truly important conceptual skills. I’m not sure they hear me, or how well they hear me, they rarely do well on these questions on my tests. If you can try to go back to the very basics to teach an adult with a clean slate mind, how would you approach these topics, and which concepts in physics are most important for them to know?

There’s been a good deal of research on how best to get people to learn the basic principles of physics, and at least among the people I’ve heard from, there seems to be a clear consensus: The biggest obstacles to learning physics are the pre-existing misconceptions of the students, and the best way to teach them is to confront those misconceptions directly.

The best examples I have for the intro mechanics class use electronic sensors connected to the classroom computer system, that I can project up onto the screen in the front of the room. This may mean that these are unworkable for people at institutions without these resources, but there are books out there with some ingenious low-cost alternatives– I’ve gotten some good stuff out of Robert Ehrlich’s Turning the World Inside Out and Why Toast Lands Jelly Side Down, and there are lots of other resources out there. I’ll describe one of the high-tech versions after the cut, because it’s a nice illustration of the basic process.

When dealing with Newton’s Third Law, the classic misconception question is something like a collision between a heavy truck and a light car, with students asked whether the truck exerts a bigger force on the car than the car on the truck, or vice versa. A lot of them will assume that the truck must exert a bigger force because of the mass difference.

I set up a track in the front of the room, with two carts carrying force sensors that can record the forces experienced by each cart during the collision. I demonstrate the basic set-up (without colliding the carts), and then pass out a worksheet that asks students to predict which cart will experience a larger force in a variety of collision scenarios: equal mass carts, a heavy cart hitting a stationary light cart, a light cart hitting a stationary heavy cart, two carts colliding and sticking together. I give them a few minutes to write down their answers, and then we go through the experiments.

When the sensors are properly calibrated (which is a little fiddly sometimes), it works great. Before each collision, I’ll ask students to vote for which cart will experience the bigger force, and on the first collision with unequal masses, there will be a good number who think the heavier cart exerts a bigger force. Every one of the collisions shows equal forces on the two carts, in accordance with Newton’s Laws, and some of the students are always visibly surprised.

Making them write down their predictions seems a little dorky, but is actually an extremely important part of the process. Everybody I’ve heard talk about this stuff says that making students commit to an answer and then be proved wrong is the only way to force them to really confront their misconceptions. If they’re not forced to commit, or even if they’re asked to privately consider the prediction, the results aren’t as impressive. Misconceptions are remarkably resilient– maybe Dave Munger can explain why one of these days…

As I said, there are lower-tech demonstrations you can use to serve the same basic purpose, and you can even do this with conceptual examples only. “Wheatdogg” offers some good ideas in a later comment to the same thread.

The key is really the process of forcing them to commit to an answer, and then demonstrating the correct result. That combination forces them to admit that they have misconceptions about the underlying physics, and start to correct those misconceptions.

1. #1 wheatdogg
February 9, 2006

Discrepant events do prick those stubborn misconceptions. The trick is to chip away at those misconceptions over and over again. I spent a year teaching this one boy physics, only to overhear him ask someone about the dangers of the coming pole shift! Apparently I failed in teaching this kid some important critical thinking skills. What was it that Einstein said: Commonsense is merely that layer of prejudices maid down by the time you’re 17?

Just consider this point. If the average person has trouble understanding relatively simple concepts like inertia and the normal force, is it any wonder so many reject the theory of evolution out of hand?

2. #2 Mike Kozlowski
February 9, 2006

“on the first collision with unequal masses, there will be a good number who think the heavier cart exerts a bigger force.”

That sort of stuff always seems more like a linguistic issue than an actual physics issue. They’re mentally thinking of the question as “If you were in one of these carts, in which one would you be more fucked?” and giving the true answer.

But it’s probably a linguistic issue that will cause them to screw up problems later in physics, so I guess retraining them into using impenetrable academic jargon is the only cure. Or else physics could refrain from repurposing common English words like “force” into having picky technical meanings, but I suppose it’s too late for that.

3. #3 sg
February 10, 2006

Chad, nice to see you take on this question. You make a good point in the summary para.

Wheatdogg, your second sentence is a key idea in pedagogy. I just wish that courses were less overloaded with material so that one could keep chipping away at basic conceptual problems till they are gone.

But this sentence is strange:

I try to elucidate the somewhat subtle differences between the application of a force and the just getting in the way of…

There is no term in any equation of motion which corresponds to “the just getting in the way of”. There’s no subtlety here. This concept just doesn’t belong in classical mechanics.

4. #4 Dave Munger
February 10, 2006

Your teaching strategy sounds right on, but it’s not supported by psychology research so much as education research. The idea is that the misconceptions students begin with are as powerful as whatever the teacher brings to the classroom. So just stating that principle X is true, without addressing the misconceptions the student may have, is more likely to lead to confusion than the student learning principle X, because the student doesn’t lose his/her old misconception, but instead just adds principle X to the morass.

If a college freshman has 18 years of “experience” suggesting that an object in motion will gradually slow down, then 5 minutes of lecture saying that motion continues until a force acts upon it is not going to do much to counter that belief.

5. #5 Mr. Upright
February 10, 2006

The car/truck problem is an example of confusing force and acceleration. The small car will undergo a greater acceleration than the truck.

It’s easy to see where the misconception comes from: students put themselves in the driver’s seat. They experience the acceleration of the vehicle as the fictitious “force” that throws them around in the non-inertial reference frame. The car’s driver will undergo a greater acceleration and therefore “feel a greater force” (and suffer more physical damage as a result).

The example is a good motivation for discussing noninertial frames.

6. #6 steve s
February 10, 2006

“Making them write down their predictions seems a little dorky, but is actually an extremely important part of the process. Everybody I’ve heard talk about this stuff says that making students commit to an answer and then be proved wrong is the only way to force them to really confront their misconceptions. If they’re not forced to commit, or even if they’re asked to privately consider the prediction, the results aren’t as impressive.”

Back in the day when I used to argue with creationists, I noticed that many of them were incapable of admitting even the most obvious errors when they were corrected. Being able to confront your own misconceptions is absolutely critical for good thinking.