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## 31 July 2014

### Horse Games -- Don't Play 'em.

Take a look at this cart.  It's moving to the right as it comes to a stop.  What is the direction, if any, of its acceleration?  Mr. Van Houten, what do you think?

"Um... right.  NO!  LEFT!"

Okay, which one?  Right or left?

"Left."

Why?

"Oh, so I guess it's right.  Right?"

Mr. Van Houten... don't play horse games with me, man!  Let's figure this out the right way so you don't have to guess.

In the early 1900s, word spread around Germany that a special horse named "Clever Hans" could solve simple arithmetic problems.  He would answer by stamping his foot the correct number of times. Amazing!  Even skeptics couldn't figure out how Clever Hans kept getting the answers right.  They made Hans's trainer go away when they asked the questions -- but the horse still stamped the correct answers, so no one was cheating.  Either the skeptics had to acknowledge a horse with mathematical ability superior to the average politician, or else something unknown was afoot.

The logical conclusion was that the horse was extraordinarily attuned to peoples' body language.  Those who asked Hans questions would tense up as Hans approached the correct answer... then they would sag with some emotion -- relief, for most, or resignation, for skeptics -- when Hans would make the final hoof tap.

For many physics questions, your students could have an easier time getting the right answer than even Clever Hans would.  The choices are often limited to "right," "left," or "neither."  Or perhaps "greater," "less," or "equal."  In any case, students who equate physics understanding with merely choosing the right answer -- or students who just want the teacher to shut up and stop talking to them -- can read the teacher's body language to nudge them in the correction direction.

Now, I may not be a good poker player with cards, but I'm a pretty good physics teaching poker player.  No matter my student's answer to an in-class question, I keep my face flat, and ask "why."  We establish early on that trying to read my face does no good.  I look the same for a wrong answer as a right answer, waiting for the explanation.  It's impossible to feign understanding.

When, inevitably, a student enters into the dialogue described above, I tell him -- and the whole class, if possible -- the story of Clever Hans.  "Don't play horse games with me.  Physics is not about getting the right answer by any means necessary.  It's about explaining the natural world using the facts and equations we've learned.  It's about communicating clear predictions about experiments, then verifying those predictions with equipment in the lab.  It's about refining those predictions when they turn out to be incorrect.

"But reading my face to get the right answer so I'll shut up?  That's not physics.  That's not even possible, as you just discovered.  So don't be Clever Hans.  Just answer the question as best you can, and if you're wrong, that's okay, you'll learn something."

## 23 July 2014

### Dashboard Monk -- where to find the "spring popper" toy

Remember the 2009 AP Physics B exam problem 1?  It described an experiment in which a "spring in a

small jumping toy" was compressed, allowing the toy to propel itself to a measured height.  Varying the mass of the toy caused the measured height to change.  By appropriate graph linearization, the spring's force constant could be determined.

But where to get such a toy?  Thanks to Drew and Robbie in my Tampa APSI for showing me.  Try googling "Dashboard Monk," or click right on the link.  I've seen these toys with Easter Bunnies, pastel eggs, and other themes.  But I've never found them other than serendipitously in dollar stores.  Now you (and I) know how to order these online.

Once you have a class set of these \$5 toys, you can do the experiment posed in the 2009 exam.  To vary the mass, stick some putty on the top of the toy; or, wrap some solder around the base.  How do you measure the height to which the toy rises?  Well, that was the last part of the 2009 problem!  Many ways are possible.  I think I will pose just this question one day to a class, saying "determine the maximum height of this toy with nothing other than a stopwatch."

When you actually do the full experiment, you could certainly use additional equipment to determine the maximum height.  Procedures suggested by students on the AP exam itself frequently referred to video analysis -- put a meter stick in the background and go frame-by-frame near the top.  A fellow reader joked that a student could put his forehead above the toy and see if he got bopped, moving his head progressively higher until the toy merely brushed his eyebrows.*

* No idea whether that's an apocryphal or an authentic answer.

One of the best parts of physics teaching is going to toy stores with the science department credit card in hand.  Thanks Robby and Drew for showing me where to shop.

## 18 July 2014

### Open Lab 2014 -- last call

Folks, the 2014 Open Lab begins a week from Sunday.  On July 27, at least ten physics teachers will descend upon Woodberry Forest School to discuss whatever is on your mind.  I've got plans to show and develop some in-class lab exercises, work with my new Vernier and Pasco rotational equipment, take an after-lunch trip to the hardware store to see if we can build something cheap and useful... mainly I'm looking forward to hanging out with interesting physics teachers in my classroom and around town.

If you're planning on coming, I need to know right away -- I'm leaving town for an AP Institute shortly, so I'm making nearly-final preparations.  I won't turn you away even if you show up last minute, of course.  However, you might not have a seat at our Sunday night dinner, a nametag, or a seat on the minibus.

Remember, the Open Lab itself is free; but you need to arrange your lodging (I recommend the Holiday Inn Express in Orange), and you need to be prepared to pay for a couple of meals while you're here.  We start officially at 4:00* on Sunday, and we're done officially at noon on Tuesday.  I can also be available a few hours before and after these official times.

* 4:00 P.M, not A.M..  I'm crazy and dedicated, but not THAT crazy and dedicated.

If you can't make it in 2014, I'll likely run something similar again in 2015 if things go well this year and if there's interest.  Just let me know...

GCJ

## 14 July 2014

### Direct Measurement Videos

Youtube has been a staple of the physics classroom for a while now.  There's no shortage of interesting videos to analyze, some of which are specifically created for science purposes.  My favorite of the produced-for-science infotainment genre is probably the Smarter Every Day series.  Take a look at the episode about how cats land on their feet.  It's got amazing footage, an entertaining host, high quality production, solid physics, and CATS!  I wouldn't use this video as a teaching tool, though.  My in-class demonstrations and labs are never designed purely as a show.  Everything I set up in class has a quantitative predictive element.  My students will certainly enjoy Smarter Every Day, but as a supplementary extra-fun part of learning physics, not as something integral to the course.

So can a video series possibly be integral to an introductory physics course?  Yes.

Take a look at Direct Measurement Videos, a series produced by Peter Bohacek, Matthew Vonk and several other Wisconsin physics teachers.*  Each video in their library shows live footage of an experiment.  Post-production work provides enough information to make, well, direct measurements.  For example, the frame number and frame rate are displayed prominently.  Where appropriate, a length scale is superimposed on the action.  Multiple camera angles are shown when useful.

* The full credits on the site list Peter Bohacek, Matthew Vonk, Ellen Iverson, and Karin Kirk.  I mention Matthew in particular because he was my table leader at the AP Physics reading.  Most of the videos seem to be credited to Peter.

While the production quality is solid, Direct Measurement Videos are emphatically NOT edited for infotainment purposes.  You won't see a narrator.  Many of the clips would seem humdrum to the non-physicist: a car braking on an ice rink, a marble colliding with a wooden block, a doll rotating on a turntable.  The excitement of these clips is that they bring to life the end-of-chapter problems that we've been assigning for decades.

Aside:  I got myself in trouble at a consultant meeting when I cheeked a non-physics-teacher presenter.  His brief speech was full of enthusiasm but empty of substance -- I wanted to get back to talking physics teaching.  The guy kept going on about how we as consultants should emphasize real-life physics.  So I raised my hand and asked him, "Could you please give us an example of physics that is NOT 'real-life' physics?"  The reaction was as if I had cited Adam Smith at a 1980 Moscow State University economics department meeting.

Of course all physics is "real-life."  The central tenet of my own teaching has been to highlight the experimental nature of physics problem solving, to the extent that I refer not as much to the "answer" to a problem as to the "prediction" made by the problem.  I set up quantitative demonstrations that have little wow-factor, but which verify the prediction made by in-class example problems or homework problems.

Direct Measurement Videos have taken that philosophy to a whole new level.  In class, I'm limited to the equipment I own, the space in my classroom, and the tools (such as Vernier's live computer data collection) that allow for immediate analysis.  DMVs have no such limitation.  They can show a roller coaster at 200 frames per second -- I have no nearby roller coaster.  They can show the slow-motion, frame-by-frame results of a dart sticking to a cart -- while I can do that experiment, I analyze with Vernier motion sensors only, and my students can't repeatedly rewind the live action to see the moments before and after collision.

My intention is to use a Direct Measurement Video about once a week as a homework problem in a different sort of "flipped class."  Traditionally I've assigned a textbook-style problem for homework, and then we've set up the physical situation in class.  Now, though, I can have the students work through a textbook-style problem in class to make a prediction, then assign the data analysis off of the video as homework.  Or, rather than assign a problem which gives all relevant input data, I can link a video and say "determine the coefficient of friction between the doll and the surface."  The students have to do more than just make a calculation; they have to figure out what data must be acquired to make that calculation, and then they have to figure out how to acquire said data from the video.  We can get away from the idea of physics problem solving as putting given numbers together in the right equations; we can get away from the idea of laboratory as a separate, distinct, disconnected portion of a physics class, instead integrating predictive and experimental physics on an everyday basis.  Wow.  Thanks, Peter, Matthew, et al.

## 08 July 2014

### 5 Steps AP Physics 1 is out...

The 5 Steps to a 5: AP Physics 1 book is out.  You can buy here via amazon.  I've completely rewritten the text from previous Physics B version because, well, the AP Physics 1 exam is completely different.  Last week in Barnes and Noble, I couldn't find any other prep books for the new exam.  And I don't trust the competing companies' books, anyway.*

* I suspect they're all superior to Barry Panas's "1 Step to a 1," which offers a money-back guarantee.

I really like the approach to content in this year's book.  Instead of textbook-style exposition, example problems, and practice problems, each of the content chapters intertwines exposition and examples.  In the spirit of the new exams, the "examples" don't pose a particular problem to solve -- getting hung up on the exact correct answer rather than the process and explanation will be a particularly harmful bugaboo on this new style of exam.  Rather, the examples pose situations which are fertile ground for all sorts of kinds of problems.  Then, I suggest what kinds of questions could be spawned from each situation, and I show how to answer each of these questions.

The book includes a variety of practice questions in each content chapter, and a complete practice exam written directly to the AP Physics 1 learning objectives and science practices.  And already I've found the first major error... that's inevitable, of course, in a first edition.  Even though I worked through everything multiple times, the first time I read the published version I found the stupidicism.  In the forces chapter, I ask a question about a block pulled along the ground by a string angled up 30 degrees above the horizontal.  Great... except that in the solution I state that the normal force on the block is equal to its weight.  Duh.  Since the string force has an upward vertical component, the normal force on the block must be LESS than the block's weight.  Grrr.  I'll have to rewrite that one for the second edition.

Any other comments, ideas, complaints, or suggestions can go in the comments, or can be sent to me via email.  Enjoy the book

## 06 July 2014

### Physics 2 Argument: What happens to the water level? Answer in the comments.

In the picture you see a 200 g mass floating inside a very light cup.  The water is inside a large beaker.  The water level is marked on the side of the beaker.

I'm going to remove the 200 g mass from the cup and place it back in the water on the bottom of the beaker.  How does the new water level compare to the marked level?  Give your reasoning in the comments.

In the most recent post, I explained about arguments and the new AP Physics 1 and 2 courses.  We absolutely must get our students discussing physics, arguing about physics, making errors and catching errors.  I personally think in terms of "physics fights", the ritual debates over research-style problems that underlie the US Invitational Young Physicists Tournament.

In order to argue about physics, first we've gotta have something specific to argue about.  We need problems that are simple enough to be accessible to first-year physics students, problems that are within the scope of AP Physics 1 and 2; but also, these problems must be complex enough to, well, produce reasonable and legitimate disagreement about physics. Arguments can't be artificial.  If you present a ridiculously bogus line of reasoning to your class, they not only won't engage, they won't even buy in to the necessary process of discussion.

My primary piece of advice is to be flexible in your teaching.  When a problem organically provokes a good discussion, go with it!  Engage that authentic argument until it's resolved, even if you hadn't anticipated it.  Similarly, if a problem that you expected to be tough gets the whole class nodding their heads in agreement, just move on.

Last week at my Mahopac, NY institute, the problem above provoked an unplanned but long and deep discussion.  I heard four different lines of reasoning from the participating physics teachers.  Five minutes of talking amongst themselves failed to resolve anything.  So we kept talking.  It was tough for some of the teachers to articulate their reasoning; others articulated clearly, but didn't convince their colleagues.  I'm an old debate coach... I noticed how a bunch of teachers knew that an argument was incorrect, but couldn't address precisely why it was incorrect; all they could do was reiterate their own argument.  (In debate we call that a failure to "clash.")

Of course, what I love about physics over debate is that once the discussion petered out, we just did the experiment.  Nature is the ultimate judge, not nine political appointees.

So what's your answer?  Please post a comment with your line of reasoning.  Don't be afraid to be wrong -- all Jacobs Physics readers are teammates, we all love each other like brothers and sisters.  The only justification I don't want -- for now -- is "I did the experiment, and this is what happened."  Comments are moderated so we don't get linkspam, so it'll take a few hours for me to post them.

GCJ

## 03 July 2014

### Arguments must be part of your AP Physics 1 curriculum

The Curriculum Framework for the new AP Physics 1 and 2 courses describes a multitude of "science practices" that will be tested across all topics on the exam.  Several of these practices call for students to discuss and argue about physics principles.  Quoting from the Framework, students are expected to, among many other things:

* refine representations and models
* pose, refine, and evaluate scientific questions
* evaluate data, both the source of data and the evidence provided by data
* evaluate alternative explanations for phenomena, and articulate the reasons explanations are refined or replaced

That's awful highfalutin' language there.  What does that mean about the questions on the actual AP exam?  Well, check out free response question 3 on the released example physics 1 exam.  It presents two different arguments -- each about three sentences long -- about whether bulbs in series or in parallel will be brighter.  The question then asks for an evaluation of the correct and incorrect portions of each argument, followed by a determination of which set of equations supports each side of the argument.  When physics teachers first encounter this question many say, holy smokes this is tough, how am I going to get my students prepared to answer questions like this?

Have them create, support, reject, and engage with arguments in class, both with you and with their classmates.  They must have experience in stating dispassionately and clearly what parts of a statement are correct, and what parts are fallacious, with clear justification that doesn't merely repeat the arguments.

How do you do that?  Practice.  Start by having students grade each others' problems to a rubric -- and not just problems that call for straightforward calculation.  Make them grade verbal justifications.

A very simple rubric for grading a verbal justification might award one point for clearly stating a relevant fact or a relevant equation; one point for connecting the fact or equation to the answer; and one more point for the answer itself.  Focus your energy in class on helping the class understand what it means to logically connect a fact or equation to an answer.  "Because of ohm's law" doesn't say anything, but "because in ohm's law with constant voltage, current and resistance vary inversely" does explain why a larger resistor might take less current.

Then create situations in which students engage in discussion with each other.  Many of these discussions happen naturally when you assign deep problems and encourage students to work together.  On homework, I consider it my role not to tutor students, but to referee their disagreements when they have differing views on a problem's solution.  When someone comes to me for help, I ask to see what he's written, and I ask with whom he's discussed his issues.  The expectation, as stated to the class from day 1, is that students go to each other first for help before they talk to me.  Not because I'm cruel or lazy, but because they must develop the skills of posing, refining, and evaluating scientific claims.

Finally, ask students to state their competing claims and debate publicly with classmates.  The debate team continually engages in head-to-head evaluation of contentions; similarly, allow class members to present their ideas to the class for evaluation.  As you referee a public debate, you have to be very careful to eliminate posturing.  While we want to find the right answer, the goal is the search for the truth -- the goal is NOT to thump chests about who was right and who was wrong.  Overly enthusiastic displays of emotion should not be allowed, whether that emotion is positive or negative.  That is, the student who gets angry at himself or who ribs a classmate for a wrong answer must be chided; but so must the student who pumps his fist and gives a Marv Albert "Yes!" when he's right.  Physics is complex enough that over the course of the year virtually everyone will be right sometimes and wrong sometimes.  When you're teaching scientific argumentation, make sure that who is right and who is wrong becomes irrelevant.  What matters is that everyone can articulate which were right and which were wrong, and why.