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Reflections on Matter and Interactions [Uncertain Principles]

Saturday, November 12, 2016 6:57
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I’m teaching introductory mechanics for the umpteenth time, using the Matter and Interactions curriculum, as we have for a while. This is going to be my last time teaching out of M&I, though, because last year the department decided to switch to a different book. Starting this winter term, we’ll be using Halliday, Resnick and Walker.

My physics blogging over at Forbes tends toward the contemplative anyway, for a variety of reasons, but knowing that this is the last time through M&I has had me thinking even more along those lines. Thus, recent posts on really simple physics and what it means to say something is fundamental and missing pieces. A detailed discussion of the pros and cons of the book is a little too inside-baseball for Forbes, though, so I’ll blog about it here.

First and foremost, I should say that the switch away from M&I to HRW is not one I would’ve made. I like M&I a lot, and enjoy teaching from it. Several of my colleagues really detest M&I, though, and haven’t gotten any happier about it over the years, so I understand the reason for the change. I’m really going to miss this book, though I suspect that the things I really like about it are also major factors contributing to my colleagues’ dislike of it.

The first thing I like, as mentioned in the missing pieces post, is that the curriculum has a very strong and definite narrative to it. They’re coming at the subject matter of introductory physics from a very particular point of view, and the sequence of topics and their presentation are carefully tailored to reflect and reinforce that point of view. It’s a very physicist-y way of looking at things, and emphasizes computation, always starting with discrete elements and repeated summation.

This sense of narrative is both good and bad– the book has a nice flow, and constantly reinforces its main ideas, but it requires some buy-in. And if you can’t adapt to their viewpoint, teaching from it is very awkward– the order of topics in electromagnetism initially struck me as strange, and I tried to do an on-the-fly re-ordering of the material the first time I taught it, with very limited success. In subsequent terms, I made more of an effort to go with the book as it was, and that worked much better.

The particular structure they choose also leads to some limitations in the material they cover. From the very first chapter, they emphasize the use of three-dimensional vectors for all physical quantities, using a very consistent convention for the axes where y is vertical and x and z horizontal. This choice of fixed coordinate system makes it awkward to do the traditional block-on-an-inclined-plane problems, though, where the key step is choosing to deal with coordinates defined by the plane, rather than by gravity. The end result is that they basically don’t do those problems at all, which is cause for rejoicing for some students, but also misses the key idea of the arbitrariness of the choice of axes.

They also have a weirdly perfunctory chapter on collisions, which sets up the difference between inelastic and elastic collisions, but doesn’t provide useful problems regarding the latter. Again, I think this is rooted in the consistent use of three-dimensional vectors, because elastic collision problems in more than one dimension are really hard to do at the intro level. (In the HRW-type curriculum, I usually skipped 2-d elastic collisions due to time constraints; with M&I, I often skipped the entire collision chapter, because they don’t get anything out of it but a couple of vocabulary words.)

On the other hand, it opens up other classes of problems, and they do more with air resistance than most texts, because those kinds of problems can only be solved numerically. As I said in the missing pieces post at Forbes, the complex-systems problems are great, and in the electromagnetism course they do some rally cool stuff using a microscopic picture of current that I haven’t seen elsewhere. So, you know, you win some, you lose some.

So, I like the narrative aspect, but it makes the text harder for a lot of faculty to adopt, or perhaps “adapt to” would be a better term. If you don’t find their viewpoint congenial, and aren’t able to fully buy into it, you’ll end up fighting against the book a lot of the time, and that makes for a frustrating teaching experience.

The other big factor that I like that puts some people off is that it doesn’t look like high-school physics. We used to teach out of HRW and its isomorphs, back when I started at Union, and I noticed a pattern in those classes where the students with the best preparation coming in would follow a particular bad trajectory. The first 5-6 weeks of the intro course on the HRW template looks almost exactly like a good high-school class: kinematics in 1-d, Newton’s laws, projectile motion, block-on-a-plane, etc., so students who had a good class in high school could pretty much coast on what they learned in that previous class. As a result, I’d see their study habits and general engagement drop off. And then when we hit them with genuinely new stuff in work and energy– dot products and integrals– they’d go all to pieces, because they’d fallen out of the good habits that would let them grapple with and power through new material.

M&I breaks that by not looking at all like high school physics. They start off with momentum– in the relativistic form, even– and everything is in three dimensions right from the get-go. Even the language used to talk about the physics is different– they don’t discuss “Newton’s 2nd Law” but rather “The Momentum Principle,” and frame the discussion not in terms of formulae for particular special cases, but iterative application of general principles.

This is confusing, even for the good students, and they can’t fall into coasting on their high-school classes. Which reduces the problem of study-habit atrophy, forcing them to stay more engaged even in dealing with material that they’ve seen before.

The down side of this, of course, is that it’s confusing, and that leads to a lot of questions from students, and a fair number of complaints. Which can be kind of a hassle, from the faculty side.

My personal feeling is that the extra confusion is actually a good thing, on balance. Others see it as being a sign that the curriculum doesn’t work for our students. There’s also some concern that students without a good background are at a big disadvantage, but I don’t really buy that. My impression over umpteen iterations of the class is that the least prepared and most confused students are no more bewildered under M&I than they were with HRW and isomorphs, but the students with good preparation are more confused, and more vocal about that confusion. I think they ultimately end up “getting it” at least as well as they did under the more standard curriculum, but they need to work at it more, and that will be to their ultimate benefit.

So, as I said, I’m happy teaching from M&I and wouldn’t switch books at all. I definitely wouldn’t pick HRW as my first choice of new text. But I understand that others have a strong negative feeling about M&I, so I’ll go along with that. There are even some benefits– we’re closing in on the point with my current approach where I would need to blow up my notes and start fresh just to keep myself from getting bored, and this forces my hand. And it will make it much easier to crib materials from the Colorado PER group (which I’ve been doing already, but carefully selecting problems and altering wording to fit the M&I language). And while I think HRW is about the most boring choice we could’ve made, that’ll force me to find new ways to keep the class from looking so much like high school physics that the students get bored and tune out. I’ve got some thoughts along those lines already.

Still, I’m going to miss teaching out of M&I. It’s been fun, and I’ve learned a lot about intro physics from taking this different perspective on it.


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