Everybody’s abuzz about the article by Paul Bloom and Deena Skolnick Weisberg (the link goes to a reprint at Edge.org; you can find an illicit PDF of the Science article if you poke around a little) about research into why people don’t automatically believe scientific explanations. From the article:

The main source of resistance to scientific ideas concerns what children know prior to their exposure to science. The last several decades of developmental psychology has made it abundantly clear that humans do not start off as “blank slates.” Rather, even one year-olds possess a rich understanding of both the physical world (a “naïve physics”) and the social world (a “naïve psychology”). Babies know that objects are solid, that they persist over time even when they are out of sight, that they fall to the ground if unsupported, and that they do not move unless acted upon. They also understand that people move autonomously in response to social and physical events, that they act and react in accord with their goals, and that they respond with appropriate emotions to different situations.

These intuitions give children a head start when it comes to understanding and learning about objects and people. But these intuitions also sometimes clash with scientific discoveries about the nature of the world, making certain scientific facts difficult to learn. As Susan Carey once put it, the problem with teaching science to children is “not what the student lacks, but what the student has, namely alternative conceptual frameworks for understanding the phenomena covered by the theories we are trying to teach.”

This has created a lot of discussion, as if it’s new and surprising information, but honestly, as a person working in physics education, my reaction is pretty much “Yes, and…?”

This is old news in physics– the article even cites a study that found large numbers of people opting for an Aristotelian picture of motion in which rolling balls leaving a curved pipe continue to travel in a curved path. The study in question is from 1980, which is probably before some of the people commenting excitedly about the new article were even born.

The new twist here is that they apply this argument to politically charged topics like evolution. But really, if people have a hard time shaking preconceptions about physics that can be proven false with about two seconds’ worth of experimentation, why is it surprising that they don’t immediately accept biological processes that take place over generations?

This does, however, remind me of something that always bothers me about these discussions, which is that they tend to be loaded with assertions that the basic rules of physics are somehow utterly alien to everyday experience. I’ll buy that for quantum mechanics, but I just don’t think it’s true for classical physics.

I often think that the problem with a lot of these conceptual issues in physics is not with the concepts themselves, but in the transition between the physical situation and the printed page. As the article notes, people actually have a pretty good innate understanding of the basics of classical physics– you couldn’t function, otherwise. The issue, it often seems to me, is not in the concepts themselves, but in the abstraction necessary to translate the real physical situation into an answer to a test question.

i-77171b2c7aca4fcf0fec1a53ae8e2856-q12.gifTo take another classic example from physics pedagogy, consider the picture at right. This is another classic question, in which students are asked to choose the correct path followed by a projectile fired horizontally off a table. The correct answer is “B,” but a fair number of students will go for “D,” which is basically the Wile E. Coyote option: it hangs in the air for a while, and then drops very suddenly.

If you really believed that the world worked that way, though, you’d never be able to catch a baseball in the real world. Most of the students who miss that question have no problem predicting the motion of real projectiles, though– if you throw something at them, they can catch it, or at least make a credible effort to catch it. It’s not that their intuition about how the world works is wrong, it’s that they don’t do a good job of making the transition from intuition to conscious thought. When they track a real projectile, they’re not thinking “This will follow a parabolic trajectory due to the constant acceleration of gravity…” They just track it, and catch it. The problem comes when they have to think back about what they did.

It’s sort of like those spatial relationship tests where they show you an odd shape of connected squares, and ask what sort of three-dimensional figure it will fold up into. Lots of people score very badly on those tests, but it’s a rare individual whose spatial skills are so bad that they couldn’t fold the actual figure up into the shape in question. The problem isn’t with the physical process, it’s with the abstraction of that process.

I often think that the process of teaching introductory mechanics isn’t really about breaking down incorrect intuitions (as is often said) but rather about bringing conscious mental processes into better alignment with existing physical intuition. Students pick the Wile E. Coyote option not because intuition tells them that’s how the world really works, but because intuition means that they haven’t had to observe the real world all that carefully, and are basically guessing based on vague recall of falling objects. They’re overthinking the problem, not over-reliant on flawed intuition.

Other times, I think that I can’t possibly be right about that, given the large number of smart people who have made careers out of studying this stuff…

I’m not sure how you’d really go about testing this, though. It would probably have to involve some sort of real physical mock-ups of the classic conceptual questions– giving people coiled hoses, and asking them to align the hose to direct water at a target, and that sort of thing. It might be fun, but it’d probably get messy, too. Especially if I’m actually wrong.


  1. #1 Donald Wolberg
    May 31, 2007

    Interesting, disturbing, worrisome, and….in the owrods of my late Grandma, “so what else is new?” I think there is a more significant sociological-educational issue. By and large, few people read any more and are not curious about science, except in a completley superficial way, and really care little for most intellectual pursuits, whether it is history, philosophy, or “gardening.” The recent up-blips in science: race to the Moon, save the oceans, dinosaurs, and now global warming, were/are relatively miniscule in terms of numbers of people interested, short-lived (except for dinosaurs perhaps), and almost completley superficial. The reasons for this are not easily provided. In part, there are so many more ways for instant gratification that require little investment in time or thought, and the amazing lack of interest in reading. Most people do not know, for example, the Darwin’s “Origin of Species,” published in 1859, was a best seller and sold out almost immediately. I do not think that a book such as “The Origin…” would be so widely sought and read today, by such a wdiverse audience.

    If I sound glum about all this…I am.

  2. #2 evangoer
    May 31, 2007

    This is a great post.

    People often say that quantum mechanics is “weird” and “counter-intuitive” — and they’re right, if you’re not someone who’s been studing QM for a few years and built up your physical intuition.

    But what people forget is that Newtonian mechanics can be really weird and counter-intuitive too. The trajectory picture you constructed is an oldie but goodie — and it’s not too hard to come up with simple Newtonian problems that can befuddle senior physics majors (or even the occasional unwary grad student).

  3. #3 Ponder Stibbons
    May 31, 2007

    I disagree. I think the Aristotelian idea that moving objects require a force to keep them moving is much more intuitive than Newton’s first law. The idea that heavier objects fall faster than lighter objects is also intuitive: that’s why Galileo’s supposed experiment from the top of a tower was so remarkable.

  4. #4 Robert P.
    May 31, 2007

    PER guru Joe Redish emphasizes that students who come into physics do not generally have a coherent alternative picture of how the physical world works. They have bundles of loosely connected or unconnected schema that they apply in different situations. Thus it’s not that they come in with an Aristotelian world view (which is coherent) – they come in with a bunch of ideas some of which may be aligned with Aristotelian hypotheses; but in other contexts they may well make use of ideas that are more aligned with Newtonian principles. This seems to me to be consistent with Chad’s observations.

  5. #5 Mike Molloy
    May 31, 2007

    If you really believed that the world worked that way, though, you’d never be able to catch a baseball in the real world….It’s not that [your] intuition about how the world works is wrong, it’s that [you] don’t do a good job of making the transition from intuition to conscious thought.

    I disagree here, although regrettably all I have by way of “evidenc” is anecdote. But I think it is a widely recognized anecdote….

    The anecdote is that baseball players typically give descriptions of the paths of thrown balls that are just not physically accurate; specifically, pitched balls. The most common report on these lines is that (good) curve balls seem to “drop off the table” when they get near the batter; that is, batters percieve the balls more or less as in line D in the figure, staying more or less on a plane parallel to the earth for most of the trip to the plate, then diving suddenly near the plate.

    I’ve also heard, with respect to pitchers with especially good fastballs, that the ball seems to rise at the batter swing, which of course it doesn’t. They used to say this of Pedro Martinez back around 1999.

    So it seems as though the perception of balls in flight is actually inaccurate in a way that matches the Wile Coyote physics model.

    FWIW, I can vividly remember my astonishment about Newton’s first law during 10th grade physics class. It had been presented to me before, but when my physics teacher applied it to some particular example–throwing a snowball off the top of a steep hill, or something–I remember being struck for the first time by how sharply the first law contradicted my naive intuition, and that it was due to gravity, not due to some inherent “stopping tendency” of thrown abjects, that the ball would quickly come to rest.

  6. #6 yagwara
    May 31, 2007

    It’s important to know what the unspoken assumptions are. For example, if the ratio of mass to drag is low, then (C) or (D) might be a better match, e.g. forcefully throwing a wadded up tissue. And frisbees can match (E) pretty well, depending how they’re thrown. Of course one may reasonably assume that the question does not concern a frisbee cannon. But the cannon might be underwater…

    On that point, #5, how sure are you that baseball players are mistaken? I’ve never looked into it, but it seems at least plausible that a skilled pitcher could get drag to accomplish some odd things.

    A Japanese mathematician (Hironaka?) was talking to his young daughter about addition, using rice balls as an example. All went well until they got to subtraction, and she insisted repeatedly that taking away 1 from 4 leaves 2. After much questioning, he finally understood that these particular rice balls are very sticky, and it is impossible to take one away without a second coming with it.

  7. #7 alpinekat
    May 31, 2007

    I agree that mechanics can certainly be unintuitive. Last Saturday, my roommate was convinced that force could not equal mass times acceleration. His reasoning was that a car crashing into another car has a negative acceleration, but it exerts a positive force on the other car. So it had to be velocity, because velocity is positive.

    I probably spent five minutes trying to explain that the second car exerted a negative force on the first car, but the first car simultaneously exerted a positive force on the second. In the end, he said he believed me, but he still didn’t “get” it.

  8. #8 CCPhysicist
    May 31, 2007

    Three quick observations:

    1) The problem from early 1980s research about misconceptions concerning motion (at least the one I have seen) used a drawing showing the different paths a ball might follow after leaving a curved pipe. I don’t see where printing it has any effect whatever on the choice being made. You could just as well set up the demo and ask people to put their hand or a cup where they think it will end up. Might make a great lecture-demo.

    2) Path D is actually closer to what you need to do to catch a softball fired from that gun than B is, because of the effect of drag on the trajectory. Drag makes it fall more vertically at the end, as anyone catching a fly ball knows. It is, IMO, the effects of friction that lead to many of the misconceptions students have. Read Newton and you will see that he spends as much time on these distinctions as Hewitt does in his physical science textbook. There is also the effect of mentally removing the effect of gravity, as noted about hitting a baseball.

    3) I always found QM to be highly intuitive. There is no accounting for taste, or that I first learned it from a guy with a PhD in Philosophy as well as one in Physics.

  9. #9 Torbjörn Larsson, OM
    May 31, 2007

    So it seems as though the perception of balls in flight is actually inaccurate

    Yes, our perceptions of what we see and do are inaccurate as well. Biologically it doesn’t make any difference if our perceptions ‘fool’ us, as long as we compensate. There is that great example with glasses that reverse the field of view – after a while people learn to invert the view back.

    I guess I am not so optimistic about the over all possibility of aligning with inherent intuition. Folk models can be the darnedest things. ‘Free will’, my ass! ;-)

    it’s a rare individual whose spatial skills are so bad that they couldn’t fold the actual figure up into the shape in question.

    I am not so sure about the applicability of such observations. People can have difficulties doing this as mental manipulations, but have a good strategy for solving such problems, including amounts of trial and error. AFAIK people who scores high on IQ tests are usually faster with solving knots, et cetera.

  10. #10 marciepooh
    June 5, 2007

    The problem of taking a mental (and often subconscious) process into the abstraction was one I had teaching unit conversions in chemistry and physics. All of my students could tell me how many quarters it took to make $2.50 but were baffled when I wrote it as an equation ($2.50*(4 quaters/$1)=10 quaters). I think there were some math literacy and math phobias at work, as well as (my own) poor teaching. (Math is a perfect way to describe these things – isn’t it obvious? Obviously not.)

    The cannon ball, and baseball, problem is complicated by drag. So is the velocity of falling objects – try convincing a room of 15 yr-olds, that just because a balled up piece of paper hits the ground faster than the flat sheet, this does not disprove Newton!