### Inertia?

Each year I start off with a mathematical modeling unit involving the classic pendulum lab: "What factors affect the period of a pendulum?" It is a simple system we can use to take measurements and graph data, and use the data to come up with an equation. And each year we start with brainstorming session. I ask the students to come up with ideas about which factors affect the period of the pendulum. They provide the standard fare: "weight", "length," "starting height," "wind resistance," "gravity," "starting force," etc. We pare it down with a discussion of what we can measure and what we can control. Wind resistance is eliminated: we can't vary that in a regular way. Gravity is eliminated: field trip to the moon is not in the budget. "Weight" becomes "mass", and "starting height" and "starting force" both are subsumed under the "angle" category.

This year, we had a new entry: "inertia." Hmmm. So it's the second day with these bright juniors, and I don't want to squash their feelings right away, so I entertain all ideas. But this one needs clarification. Clearly this student knows some "physics lingo." So I queried,"What do you mean when you say 'inertia?'" I say it with sincere interest, emphasis on "what do

__you__mean." The student stumbled a little bit before continuing, "Well, you know, when you start it swinging, and it goes back and forth for awhile, decreasing slowly each time until it comes to a stop? Inertia." Oh, right "inertia" - almost exactly the opposite of what physicists mean. So at that point, I'm forced to disappoint the student, and very briefly explain what physicists mean when we use the word "inertia" and how we will discuss that later. And what the student means is something else, which we are going to ignore here by not letting the pendulum swing for too much time.
Or maybe it's more subtle than that. Part of the definition of inertia is the tendency of objects to remain at rest. So the pendulum slowing down and stopping: it's tending to go back to being at rest. That's inertia, right? Alas, no, it is not.

I am thus reminded that these students not only do not come to me as blank slates, but they come chock full of misconceptions and partially formed ideas, heaped on them by movies, TV, news and by well-meaning, poorly-trained, over-worked (and, no doubt, intelligent) middle school science teachers. So many physics words have entered the mainstream that I can't keep track of them any more. "She's a real force to be reckoned with." "The stock market lost momentum today." "You shouldn't make such a quantum leap in your reasoning." And that's just physics words appropriated by other disciplines. What about the mistakes people make when trying to apply the physics correctly? As I say to my students, "Don't get me started." However these misconceptions are planted, I need to confront these them head on, or they won't go away. I am reminded to not use words they don't know until they have a concept that needs a word. Concept first, name later. Otherwise we are just teaching vocabulary…with math. And in AP Physics 1 (formerly known as Honors Physics), we have LOTS of math. Especially the most frightening of math: algebra!

If I don't want my physics class to be reduced to vocabulary plus math, I have to strive daily for the ever elusive "deep conceptual understanding." I have to search out those misunderstandings. I have to stop myself from dismissing a student's "silly answer" and really listen while they explain what they are thinking. And then (this is the tough part) I have to devise a way to show them a new way of understanding. A way that will stick long beyond the unit test, all the way to the final exam…and maybe even until they get to college. Here's where it gets really fun: there are 24 students in the room. [Yes, I'm spoiled that way. Small classes.] And they each have their own preconceptions. And in 42 minutes they leave, and 24 more walk in the room with a whole new set of preconceptions.

Fortunately, I know most of these preconceptions*, and I usually have a plan for those. But every so often there's a new one. Like inertia. And I have to really think on my feet. That's the challenge despite teaching the same thing every year: someone, some day not too far in the future will walk into my classroom and reveal a way of thinking about something that I haven't encountered yet. I get to meet them where they are, and walk them over to a new way of thinking. I get to use toys and demonstrations and labs and ranking tasks and humor and compassion and, for the really bright ones, I get to use algebra. That's what we in the business call "phun."

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**Thanks to all the other physics teachers and students who have taught me about these misconceptions during my career, and to a great book by Arnold Arons. And of course, to my dad.*

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