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28 July 2010

An AP physics problem-of-the-day

AP Physics reader Craig Fletcher made me aware of, a site run by his colleague Richard White at the Polytechnic School in Pasadena.  [No, not Cal Tech, an independent high school near Cal Tech where many professors' offspring attend.]  Richard provides -- for free -- an new AP physics-style multiple choice problem each day.  He'll even deliver his problem automatically to your email inbox.  Richard bills his service as a study aid, but I think of it just as much as a teacher's aid.  Need a multiple choice question quickly for today's quiz?  Look at his site.

I browsed a bit, and while I can't vouch for every question, everything looked quite strong -- at the correct level, doable without a calculator, with reasonable language.  The only caveat I'd attach right now is that physics B and C questions are mixed together.  Nothing wrong with that at all, just be aware.  Generally multiple choice questions from unedited sources are buyer-beware.  I don't recommend using questions from the ubiquitously available College Physics tests you can find online, for example.  But Richard's site seems like one I can recommend.

It's not like there's any real shortage of good AP-style problems out in the online universe.  But, any further suggestions, please send them along, and I'll point readers in that direction.


27 July 2010

Changing AP score cutoffs now that the guessing penalty is gone

In early June, the College Board released the news that, starting next year, the raw score for an AP multiple choice section will be the total number of correct questions, without a "guessing penalty."  You first heard it here, on the Jacobs Physics blog.

One of my major recommendations to AP physics teachers is to make ALL classroom tests in AP format.  This means use authentic AP questions, give the appropriate amount of time for each (one minute per point free response, 7 multiple choice every 9 minutes)... and use a scale converter to translate each student's grade into an approximate AP score.

In past Physics B exams, it's taken roughly 65% of the available points to get a 5, 50% to get a 4, and 35% or so to get a 3.  That exact conversion fluxuates year-to-year depending on the difficulty of that year's exam, but this was a good rule of thumb.  Problem is, now that the subtraction of 1/4 point for each wrong multiple choice question is gone, raw scores will be higher.  How will these numbers change?

The calculation is not quite as simple as it initially might look.  Scores of the weaker students will improve more than those of stronger students, because weaker students will miss more questions and thus fail to be penalized for "guessing" more often.  And free response scores will not go up, only multiple choice.  This all points to adding a bit to each score cutoff, with more being added to the lower cutoffs.

This year, I'm going to use these cutoffs in my class.  I'll see if they work... and I'll appreciate feedback, as well:

5  68%
4  55%
3  41%
2  30%

We'll only know more through experience, and when another released exam comes out.


17 July 2010

No one ever went broke underestimating students' calculational skills

Look, physics isn't about math.  We have to get that message across as many times and in as many ways as possible early in a high school physics course. 

I have already detailed my first salvo against the preconception that math is what makes physics hard.  In sum, I suggested using the minimal algebra skills necessary for AP physics B as a recruiting tool.  The two skills necessary are (1) solving simultadous equations, and (2) knowing the definitions of the three major trig functions.  "You know these things cold," I say.  "I promise, there will be no more complicated math, regardless of what you might be covering in your algebra II or precalculus class."

I received a note with an interesting idea from Matt Swanson, recently of my Kennesaw State AP Summer Insitute.  Matt says:

"I read your blog post about pre-tests and I agree with your advice. However, I was toying around with the idea of giving one on the first day of school anyway. I was thinking about giving them a two question quiz that consisted of your recruiting questions..."

I totally see where Matt is coming from.  He's facing a roomful of nervous AP students whose prejudice is that they're in for a math-style course tougher than the nastiest level of calculus.  He wants to put them at ease about their math skills, while at the same time establishing the daily quiz routine.  My initial reaction was that this might be a good idea in Matt's situation, though I would not use it myself. 

On further thought, though, I realized that such a first day quiz might easily backfire.  Why?  Because I'll bet that a significant fraction of the class would get the very basic questions WRONG.

Quick side story:  When I first started teaching Universal Graviataion, I set up the calculation of the force of the earth on the moon.  I wrote the numerical values of each term on the board, and asked the class to plug into their calculators.  Why?  I knew that a very common mistake in such a calculation is to forget to square the denominator.  My hope was that everyone would get an answer within about 45 seconds, that 17 of 20 students would get the right answer, while the other 3 would forget to square the denominator.  We could have a nice teaching moment (Don't forget GMM/r SQUARED!)  and we'd move on.

Veteran teachers are likely laughing at me in the voice of Emperor Palpatine.  (YOUNG FOOL... ONLY NOW DO YOU UNDERSTAND....)  Of course this didn't work.  It took 3-4 minutes for most of the class to get answers.  Of 20 students, I saw 17 different answers.  I tried again the next year - same result.  Okay, I said, I give up -- I'm going to teach order of magnitude estimation in class, and let the students play futilely with their calculators on their problem sets. 

I've seen similar crazy difficulty with the most basic of quiz questions involving recall or basic arithmetic skills.  As the year goes on, students learn not to fret about occasional dumb mistakes - when an AP exam only requires 65% of the available points to earn a 5, and when the final numerical answer is wholly subordinate to the physics reasoning behind that answer, who cares if a student said that 2 divided by 5 is 0.6. 

My worry about Matt's opening day quiz is that, no matter how simple the algebraic exercises, SOME WILL GET THEM WRONG.  And then the whole point of Matt's opening gambit -- look how easy the math is, you can do it! -- -will have blown up brutally.  All it would take is a few students saying, "Geez, he says that quiz was easy, but I still ran out of time and got it wrong.  I'm failing already on the first day."

Don't worry -- if you do physics from the first day of class, integrating mathematics only where necessary, students will get good at doing the math.  And, if YOU model the correct attitude that mathematical crunching is subordinate to physical understanding, the students will pick it up.


12 July 2010

iPad apps for physics teaching

At the AP reading, and again at my summer institutes, physics teachers showed me all sorts of applications for their handheld devices.  As I expressed reluctance to dive into such trendy tech gear, Karie Meyers (a college professor at my table) sat me down and told me bluntly -- mobile apps are here to stay.  Teenagers use them fluently.  Either learn to use them, Greg, or be considered a dinosaur by your students. 

Based on her prodding, and further kicked in the butt by the attendees at Kennesaw State, I asked Woodberry's tech director about iPad apps.  He had bought one iPad to share around departments, to see whether anyone could find a serious academic use for it.  I am his first guinea pig - I have been using the iPad over the weekend.  I've found apps to measure magnetic field (free), to find current and predicted weather and barometric pressure (free), to measure the angle of the iPad (free), and to show and name stars in any direction (not free, but bloody impressive). 

I want to find something to make position- and velocity-time graphs using the internal accelerometer.  Pasco makes a free app that will plot acceleration vs. time, but I can't figure out how to derive the other plots on the iPad.  (Pasco also claims that this app can plot output from ANY of its probes using bluetooth.  Great, but I have Vernier, not Pasco, probes.)

I've found other cool apps that are only tangentially related to physics teaching, like google earth, a giant timer, a ruler, a periodic table, and so on.  There are physlet-style simulation apps available, but (a) they cost money, (b) they're available online for free, and (c) I'm not a huge fan of simulations for teaching purposes, anyway.

I have little doubt that, five years down the line, Pasco and/or Vernier will have updated their data collection line such that the probes work wirelessly with the iPad or equivalent, and the LabPro or LabQuest will be unnecessary.  That's why I need to start figuring out how to use these things now.

What I want to know from your comments and emails:  What data-collection apps are available?  What apps might I use directly or indirectly in my classroom?  Give me some ideas to track down...


05 July 2010

Mail Time: Should I give a pretest?

This is Michael Gray. I was the new teacher who attended your AP Physics workshop in Kennesaw last week. I am preparing for August and I have a few questions:

Hi, Michael... great to hear from you.  Yeah, it's crazy how youall in Georgia start so danged early.

I am teaching a second year class. Would you recommend giving them some sort of prior knowledge test? And if so, how would you use it?

First, I'll answer this for one of your SPECIFIC classes -- you are picking up a second-year course taught by your competent colleague who left the school on good terms.  There's no need for a pretest.  You have much better uses for your class time. 

If you wonder what was covered last year, ask the departing teacher.  If you wonder how much the class remembers or retains, then a combination of a deep conversation with the departing teacher and observation over the course of the course should answer that question just fine. 

Try starting the year with a topic completely new to them.  After a week or so of that, you should have some idea of how much you need to discuss the things that were covered last year... and you won't have inadvertantly made bad assumptions about what they retained.  The combination of conversation and observation will be more accurate and less classtime-consuming than a pretest.

BUT FOR THE TYPICAL INTRODUCTORY PHYSICS COURSE:  You've put your finger on the crux of the question by asking how you would use the pretest.  I once had a long argument with the gentleman charged with evaluating my school's science department.  He contended that I'd do a better job if I were to give a pre-test.  I asked, "To what end?  For what purpose?  How would the results change or inform my teaching?"

"Then you'd know where the students are, and can tailor the course to their needs," he said.

I had three responses, two of which are relevant to your purposes, Michael:

1.  In a large sense it doesn't MATTER where they are, especially mathematically.  I only need basic algebra I level skills.  I don't care what they already know about, say, Newton's Laws, because much of what they know will be misconceptions born of misinformation.  In a first year course, I'm teaching everything from scratch; for a second year course, I'd strongly recommend the "conversation" approach above rather than a  pretest.

2.  The students will be at many DIFFERENT places, both in terms of mathematics, and in terms of prior knowledge of physics concepts.  Sure, maybe 1/3 of your class is mathematically astute, or 1/4 of the class had a really good 8th grade physical science teacher who taught them properly about Newton's Third Law.  Big deal.  You still have to find a way to make everyone successful and interested, whether it's 5 or 15 students at the top end of the class. 

3.  I already know where my students are, or at least I can figure it out very, very quickly.  I've been at the same school for 11 years now.  I'm getting similar classes every year.  With each new class, I can tell with reasonable precision after two problem sets who is smart, dumb, lazy, clever, or whatever.  How?  By talking to them, reading their eyes during class, reading their problem sets, and if I'm totally confused, talking to teachers who had them last year. 

Moral: I don't need no stinkin' pretest.  And neither do you.


03 July 2010

Money and Significant Figures

As those who take my summer institutes know, I do not spend any class time going over mathematics.  No vectors, no trig, no unit conversions, no significant figures.  Physics teaching literature has for years disputed the efficacy of the mathematical prelude; experience and anecdotal evidence also  leads to the conclusion that such preludes are worse than useless.  You can find my diatribe about the first day of school in this post.

Of course, the begged question becomes, do I ever go over mathematical methods?  Well, never in a way that is divorced from actual physics.  I teach juniors and seniors.  They've been taught how to convert units and how NOT to write down 15 digits from their calculator, and they've probably been taught these things multiple times. I have more important things to teach.

Yes, when a mathematical issue comes up in a real physics problem, I might mention a technique; for example, I'll show on the board how know to move the decimal 6 times to convert milliliters to cubic meters.  I'll go through the solution to a two-simultaneous-equation problem IF the problem springs naturally from a quantitative demonstration.  And I always demonstrate an appropriate use of significant figures, occasionally with a brief comment.

Last week at my AP Summer Institute at North Cobb (Georgia) High School, a participant shared perhaps the most ingenious approach I've ever heard of to significant figures and numerical precision.  Tiana Stroud, an Atlanta-area physics teacher, suggested a reference to money owed.

Say a student in the first week asks breathlessly whether he got the right answer to a problem.  "You said the answer is 50 cm, but I got 49.8 cm.  Aren't I right?"

Tiana's answer: "Okay, kid, pretend you owe me fifty bucks.  And you pay me 49 dollars and 80 cents.  Is that okay for you?  Are you going to risk our relationship over those missing 20 cents?"

I love that reasoning...

* It works for the students who complain about estimating g as 10 N/kg.  (Okay, I owe you $9.80 and I give you a $10 bill.  What would you say if I stood on principle and demanded my 20 cents of change?) 

* It works for students who write down every digit on the calculator.  (I owe you $49.891202.  I give you 49 dollars and 89 cents, and ask for .1202 cents on credit toward our next transaction.  You say I'm a bloody idiot.)

* It works when an answer is off by a good bit.  (Sure, you calculated 0.15 m, but I measured 0.10 m.  That's more than a 30% difference.  Let's say I owe you $150, but I give you $100.  Are you okay with that, or are you going to send Vinny and Guido to ask my kneecaps for the remaining $50?)

Love it.  Thanks, Tiana.  I'll possibly update you throughout the next school year as I use this analogy.