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01 June 2018

The Last Jedi - analysis of theme and Holdo, with (yes) tenuous physics teaching connections

I’m a big fan of Alastair Stephens’ podcasts.  He’s done a 12-episode series on Story and Star Wars, a 60-plus episode series on Tolkien’s works, and a long-running Harry Potter podcast.  Alastair doesn’t just give a fan’s perspective, though he is certainly a fan.  He delves into the text of a story. He commits literature on what many consider merely nerdish-pop culture phenomena.  If you like any of these titles, I highly recommend checking him out at

Alastair finally got around to talking about The Last Jedi a few weeks back.  I sent him a response to some of his analysis.  In doing so, I realized that there’s a physics teaching connection to a couple of these.  And you can’t go wrong discussing Star Wars amongst physics teachers.  

So in the spirit of love for intellectual Star Wars discussion... Here are three thoughts on The Last Jedi after listening to Story and Star Wars:

(1) Alastair spends some time searching for The Last Jedi’s overarching theme, which he points out is not at all clear.  I took the theme of the show, such as it is, to be contained in Rose's final line to Finn: "saving what we love, not killing what we hate."  Poe was demoted for violating that principle. Rey found the part of Kylo that she could love and save, not kill and hate.  Rose's trip to Canto was "worth it" only when she saved the camel creature. Problematic Holdo planned to save what she could rather than make a heroic last stand. Luke took action when he realized that while he couldn’t destroy the first order or Kylo, he could at least save Leia, Rey, and the rebels.  

I certainly see Alastair’s question about the movie’s confused, disparate, and unfocused themes.  No argument there.  But, is it possible that all the possible themes he mentioned - war/peace, leadership/heroism, harmony/disharmony, etc. - could be encompassed in saving/killing?

(And, combined with the borderline polemic scenes on Canto, I took this theme as as close to a current political message as you'll get from Star Wars.)

Physics teaching connection: In many a May as the school year draws to a close, I find myself a bit bitter.  All too often, a colleague close to me is leaving the school; or a rotten, racist student accepts his barely-earned diploma with a sh*t eating grin; or I have to play nice as an unprofessional kiss-arse of a teacher is lauded by those who don’t make the effort to know better.  

Saving and killing are, of course, supremely hyperbolic when referencing life at a bucolic boarding school.  Nevertheless, I find myself reciting Rose’s mantra to myself.  The negatives that stick in my craw are far outweighed by the positives. The colleagues I love who remain on faculty (and the new folks coming in), the myriad students who graduate with pride at a job well done, the outstanding teachers who win well-deserved recognition… I need to treasure those people I love rather than seethe at those things and people that bother me.  If Star Wars helps me to do that, so be it.

That doesn’t mean I should give up the fight to improve my school, my students, and my own teaching.  I merely have to keep in mind that losing the battle of Hoth doesn’t mean losing the war.  

(2) Alastair presented an analysis of Admiral Holdo’s actions.  Her opposition to Poe - and, to judge by the mutiny that commences, her opposition to most of the rebels below a certain age - contrasts with her heroism and her camaraderie with Leia.  

I talked to the son of a Marine officer about her. He said, "A Marine captain who demands big-picture details from an admiral in a public forum would be rightly thrown in the brig.  The captain's job is to follow orders.  Holdo's response to Poe's insubordination was mild."  

Fair point.  Yet, as a physics teacher I see her response to fair questions from her subordinate as deeply problematic.  On the podcast Alastair indicated exactly the response I'd expect from Holdo, and that I give to students who question my methods.  Something like, “Poe, please understand that we do have a throw of the dice left.  I can’t divulge the details now, because I’m worried that the First Order can hear us.  But I promise that we will not merely wait here stoically to die.  Please trust me as Leia did.”

I’d be personally okay with even, “Poe, back the eff off.  I’m using what fuel we have for transports; we’re headed to a secret abandoned base.  That’s the plan.  I’d love it if you could now please take BB8 to do the final maintenance check on every transport.  That’s an order, Captain.”

The "shut up" response should be reserved for when the subordinate's questions broach into tendentiousness or bad faith.  And even then, making direct personal insults to a subordinate - “I’ve dealt with flyboys like you” - is not only out of bounds, but the exact opposite of productive.  (What talented, prideful subordinate ever responds to a direct insult with “Oh, the boss showed me.  I guess I’d better shut up now and do what she says.”?)  A large part of leadership is effective communication and managing the personalities of talented underlings.  Holdo failed.  

Physics teaching connection: The education industry does not well tolerate people with exceptional ability, especially if those people are brash.  To mix my performing arts metaphors, administrators want their teachers to be Burrs, not Hamiltons.  And, to my profession’s shame, teachers usually want their students to be Burrs, not Hamiltons.  Especially when the teacher is more Samuel Seabury than George Washington.  (Okay, back to Star Wars references.)

What is your plan when you’re presented with a student who’s extraordinarily talented, but who is impatient and frustrated with the strictures or the pace of your class?  What is your administration’s plan to deal with the young, brilliant, popular teacher who has the political acumen of Jar Jar Binks?  You should have such a plan.  The plan should provide support and encouragement, while at the same time guiding the Padawan toward wisdom. 

If your plan is to do a Holdo, you’re in the wrong profession.  I’m rather disturbed that a number of fans I talk to blame *Poe* for the catastrophes at the end of the second act.  "If he had just shut up and followed orders..."  No, that's on Holdo.  "If she had just communicated her plan and built trust..."

(3) Mechanisms of the force: Luke's mother of all Jedi Mind Tricks - A colleague of mine put the final scene with Luke into perspective for me.  We've never seen a force hologram with such power.  What if, my colleague asked, in the final confrontation Luke is performing a Kenobi-style Jedi Mind Trick... but not just on one mind.  He's performing the Jedi Mind Trick on every single person watching.  He's convincing all of the 200 or so people present to see something that's not really there, and to see the same thing that's not really there. This would explain the long-distance nature of the projection, in that Luke must touch minds through the force, not physically project a hologram equivalent.  It would explain how Luke can fight as a hologram, and why sometimes he is substantial and sometimes not - Luke is not physically fighting, he's merely making Kylo and observers see and believe in a fight.  It would explain Luke's death better - that sort of performance took all of his remaining energy. If that's true, that would make Luke truly the most powerful Jedi ever, and even a greater legendary hero.

Physics teaching connection: There is none.  Other than that virtually all physicists have a personal understanding of Star Wars and the Force.

20 May 2018

Did you or your students get a nonstandard form for an AP Physics exam?

The vast majority of students who take an AP Physics exam get the “operational” version of the exam.  This is the one whose multiple choice questions are never released, but whose free response is available publicly 2 days after the exam date.

A small fraction of students take a different form of the exam.  In particular, students outside North America usually get this different form - otherwise, it’s possible for a particularly dirty player to reconstruct the exam and text details to a student in a different time zone.  Within the continental US, that’s difficult - the west coast has entered the exam before the east coast is finished.  In the fall the College Board releases the majority of this “international” version of the exam to teachers who have passed an official course audit.

For statistical purposes, occasional randomly-selected students stateside take the exam form that’s otherwise earmarked for the international audience.  Invariably, when a few students find out that they took a different test than their classmates did, they claim that (a) they took the harder version, and (b) no fair.

Claim (a) has a 50-50 shot of being true.  Claim (b) is utter, ignorant baloney.

Is the international version harder?
Sometimes, sometimes not.  Is this year’s exam harder than last year’s?  It depends on your perspective, what you’re personally good at, and random chance.  The development committee attempts to construct exams of similar difficulty in each year, in each form. If you have access to the released international exams, take a look through.  Some questions are harder, some easier, most about the same.  Once many years ago I was in the super secret room where the international and alternate forms of the exam are graded - on pain of pain, I’m not allowed to tell you any details about the exams I saw.  That’s just as well, because I don’t remember details.  Yet, I can tell you that the questions read exactly like the operational exams - I perceived no difference in difficulty, no difference in the range of student responses.

Are the AP scores lower for the international version?
No.  The difficulty of exam questions does not matter when AP scores are compiled!

On each exam, a subset of multiple choice questions are repeated from previous years.  These serve as a control on exam difficulty.  Even when an exam consists of demonstrably more difficult questions, the meaning of a 5, 4, 3 is identical.  This subset of multiple choice ensures that desired outcome.

Let’s say that all students taking the international version got 40% of the available points, while all students taking the operational version got 50% of the available points.  There are two possible explanations for this discrepancy:

(1) The international exam consists of more difficult questions than the operational exam.
(2) The population of students taking the international exam is weaker.

Performance on the subset of repeated multiple choice questions can differentiate between the two hypotheses.  

If there was no statistical difference in performance on the repeated subset, then explanation (1) applies.  The same level of student getting fewer points means the exam is harder.  Thus, the cutoff percentages for 5, 4, 3 would be dropped.

However, if the students on the international exam scored similarly worse on the subset of repeated questions, then hypothesis (2) is confirmed.  Even on the exact same questions, these students performed to a lower standard.  The cutoff percentages would reflect that lower level of performance.

The overarching goal is that a 5, 4, or 3 means the same level of performance from year to year.  Yes, it is true that for AP Physics 1, about 70% generally works out to a 5, 55% to a 4; those numbers are variable to serve the overarching goal.  Similarly, it is true that about 5% of the student population gets a 5 on AP Physics 1.  But that's also variable, dependent on the performance of this year's students.  There's nothing preventing teachers and students getting better year to year, such that suddenly half the country earns a 5.  I mean, that's unlikely... but the College Board would be utterly thrilled if that happened.

Scores are not manipulated for political reasons.  In the very first year the AP Physics 1 and 2 exams were administered, a diverse committee was assembled and teachers/professors were surveyed to determine the standard of performance appropriate to each score.  While statistics were used copiously, this original score setting included some subjective as well as objective input.  

Once those original scores were set, though, the standard of performance for each score was also set in stone.  The scores this year will be what they will be... if more people get high scores (as has been happening very gradually across the last few years), that represents authentic improvement in the student population taking the exam.  No one is pulling strings behind a curtain trying to depress or increase scores.  Those who promulgate conspiracy theories do so out of malicious ignorance.  The numerous ETS statisticians, the chief readers, the College Board representatives, the development committees... all these people would have to be part of a such a grand conspiracy.  I know many of these folks - I know them to be people of considerable integrity.  They wouldn't stand for fudging scores.  It's not happening.

Similarly, there's no conspiracy to give half of your school a harder exam with worse scores.  If you happened to get a nonstandard form, well, everything will come out in the wash.  Your exam score will reflect your knowledge of physics.  As it's supposed to.  :-)

16 May 2018

Chimera: The skill of learning from a textbook

I was asked whether I still use just fact sheets, or whether I encourage the use of a textbook like OpenStax.

I'm still using fact sheets, and nothing but fact sheets. 

Many people have argued with me that they want to promote learning new physics from a textbook.  Reading a textbook is a skill, one that seems to be demanded in college physics courses.  Fact sheets and videos, they argue, eliminate textbook learning.

True.  And I desperately want to eliminate textbook learning.  

Textbooks are interesting and useful and fun to read for those who already have context.  In history, where the vocabulary and general situations (kings, wars, human conflict and leadership, etc) it's easy to learn from a textbook.  Life science texts with outstanding diagrams can lead to good learning.  Even language texts can be strong, especially if they include easily navigated glossaries and grammatical summary tables.

Physics cannot be learned from a textbook, I don't think.  Instead, students can learn facts from a fact sheet by rote. Then they can learn the skills of applying those facts to interesting situations, and experimental skills in lab, and how experiment connects to prediction... and finally, at the end of the year, students can go read a textbook successfully because they have the context for understanding.  

I think that process holds in first year physics as well as advanced physics - everyone I knew struggled to understand our quantum text, or advanced fluids text, or advanced E&M text.  I thought I was stupid because I couldn't read and understand the textbook.  My friends either also thought they were stupid, or thumped their chest and pretended to understand.  

We all figured these subjects out eventually, as we did problems and learned facts from our teacher and practiced with each other.  But the idea that it's a useful or even attainable skill to read a text and understand new physics the first time you read it?  Utter baloney, in my mind.  :-)

14 May 2018

The Jacobs Physics Podcast, ep. 3: 2017 AP Physics 2 problem 2, resistivity experiment

The 2018 AP Physics 1 exam involved an experimental graph to determine resistivity of an unknown material.  Interestingly, so did the 2017 AP Physics 2 exam.  This week's podcast deconstructs the problem and rubric from the 2017 P2 problem, including digressions about:

* writing procedures
* how I feel about formal lab reports (i.e. don't do them unless you're truly dedicated to the process)
* the 1/4 page or 1/2 page rule for graphs
* how I feel about science fairs (i.e. don't do them unless you're truly dedicated to the process)

Click on this link to listen to this week's podcast.

Feel free to send feedback via email.  I've already addressed a couple of peoples' questions on the podcast; I'd love to have more.  If you want me to use your name, say so!


10 May 2018

2018 AP Physics 1 Solutions

What a great exam this year... I particularly loved #3, the quantitative-qualitative translation question in which students have to work through a non-constant torque graphically and conceptually.  The paragraph question which combines energy and momentum concepts is likely to show up at our year-end conceptual physics tournament in the near future.

My solutions can be found at this TEACHERS ONLY link.  Yes, really, teachers only.  If you ask for access as a student or parent, Gardner, who runs that site, will take away your birthday, and for extra measure, he'll lay a little spell on you right there.  He'll turn you into stone, or a dog, or a chair.

Teachers, if you'd like access, please follow the instructions at the site.  Send Gardner a request to join, along with evidence that you are a physics teacher.

The official solutions will be available in the fall on the College Board's AP central website.  I'll be grading problem 3 - woo-hoo!  (No, I've no clue yet what the rubric will look like, just that I'll be grading thousands upon thousands of problems 3.)


07 May 2018

The Jacobs Physics Podcast, episode 2: 2015 AP Physics 1 Problem 3

This week's podcast discusses the first ever paragraph response problem, the one about two identical spheres landing on the ground, and why they land at the same time.

Digressions include:

* the four minute drill
* last-minute AP prep
* how your physics class already supports goals of project based learning
* how AP rubrics are developed
* what constitutes a "paragraph response"?

Click this link to listen. 

Would you like me to discuss a particular issue on the Jacobs Physics Podcast?  Send a note via email...


06 May 2018

How do you get ready for the AP Physics exam in one night?

You don't.  Physics understanding is built over months, not days; new skills cannot be crammed at the last minute any more than just one night's worth of weightlifting right before the big game can benefit a football player.

Nevertheless, you might feel that your students aren't anywhere near ready for the exam coming up in a day or two.  What can you do?  How do you help?

Take a long-term approach.  Instead of performing triage, instead of brutalizing yourself and your students with weekend-long review sessions and late-night tutoring, conduct a tactical retreat.  Evaluate, why are your students feeling so unprepared?  What can you as the teacher do next year to avoid this last-minute panic?

Thing is, thinking about next year is not your instinct.  Your teenage students live in the moment.  This moment says, help me now while my academic life seems to be crumbling.  And you care about your students, so you help... then you and they move on in the catharsis after the exam, cleansing your collective memories of the despair and desperation youall felt this weekend.  (Or worse, normalizing those feelings as a necessary and unavoidable part of academic life.)

But you cannot live in the moment.  Your career teaching physics will span years if not decades.  Chances are that if your class is panicked, this isn't the first year they've felt this way.  By definition, it's insanity to do things the same way again and again expecting different results.  So consider how you can change... starting with the first day of school.

Your students certainly feel an urgency now, with the exam looming, to pay attention to physics, to figure out difficult concepts even at the cost of significant brain energy.  It is our job as teachers to create that urgency throughout the school year.  Use every trick in your book, and learn new tricks: give frequent quizzes, don't let students get away with half-arsed work, do test corrections, don't answer questions during tests and quizzes, enforce the five-foot rule so each student is always writing her or his own understanding, don't slow the pace of the course for anything less than the apocalypse, give less-frequent cumulative tests rather than unit tests...

Spend the next few weeks asking your students what they might suggest youall could have done to reduce the pressure at year's end.  Spend the summer building your toolbox, by reading this blog, by reading everything available on the College Board's AP Central website, by finding other physics teachers with whom to talk shop, by attending a summer institute.  Come to school in August with a well-formed plan so your students spread their panic in itty bitty, barely noticeable doses throughout the year.

Be prepared, though.  Teenagers live in the moment.  You will get pushback as you demand more effort and engagement in the fall.  It is, in fact, our job to weather that storm, to keep our students focused on long-term goals.  That means some difficult conversations early on, that means some political damage control with parents, colleagues, and administrators.  It will all pay off in May, when your students approach the impending AP exam with calm confidence.  Then the NEXT year, the positive feedback from your well-prepared students will mean less pushback in the fall.  And so on, until you have a well-understood culture of learning physics intensely but calmly, without drama or last-minute fear.

30 April 2018

The Jacobs Physics Podcast: AP Physics 1 2017 problem 3.

I recorded a 30 minute episode in which I discuss - with digressions, of course - the third problem on the 2017 AP Physics 1 exam.

You can see the problem via this link to the official College Board website.

Then, click here to listen to the podcast!

I'd love to hear your feedback; in particular, I'd love for you to tell me what you'd like me to discuss on a weekly podcast if I continue it.  You can find my email through the Woodberry Forest School website.


23 April 2018

Do I need to know about pressure/displacement nodes in AP Physics 1?

I was asked about standing waves in pipes on the AP Physics 1 exam.  Specifically, is it important for students to understand the difference between a variation in air pressure, and a variation in the amplitude of particle displacement?

On one hand, it is certainly important to understand nodes and antinodes in closed and open pipes.  When the boundary conditions are the same (closed at both ends, open at both ends) the fundamental frequency is v/2L and all multiples of the fundamental frequency can be played; when the boundary conditions are different (open at one end and closed at the other), the fundamental frequency is v/4L with only odd multiples available.  

Then, we have to understand the WHY behind these facts.  Students must be able to draw pictures of standing waves, must be able to identify the wavelength as it relates to a picture of standing waves and as it relates to a pipe length, understand why the speed of waves is constant, how the wave speed relates to the speed of particles in the material, how the particles actually move in a transverse wave and how that relates to the pictures and to the wave's amplitude.  (Whew.)

But, when drawing standing waves, are we drawing a representation of the air pressure or the particle displacement?  And, does it matter?

Any time you're wondering about what will be tested on an AP exam, be as a biblical fundamentalist: ignore peoples' pronouncements and go straight to the source text.

I've just been through four released AP Physics 1 exams.  Not a single question addressed the difference between air pressure and particle displacement.  Now, that doesn't mean a question next year or the year after couldn't do so, because the AP Physics 1 exam is only a few years old.  

Nevertheless, I'm taking a Bayesian approach.  The difference between particle displacement- and air pressure- representation is extraordinarily abstract, and difficult to understand for a student who hasn't studied fluids, anyway.  The mathematics and representations of standing waves work fine even if students don't know what exactly they're representing.  Therefore, I don't address this issue of pressure vs. particle displacement.  

If that means my students have to guess on one multiple choice question every half-decade, that's a price I'm definitely willing to pay for simplifying their understanding of standing waves in a pipe.

22 April 2018

Why is the net force on a car greater than on the driver?

A reader of my 5 Steps to a 5: AP Physics 1 book sends some kind words about the book, and then asks:

Number 2 in the Forces and Newton's Laws review chapter is about the net force on a driver vs. a drag racer.  To calculate the net force on the drag racer you omit the mass of the driver.  I was wondering about the logistics of that.  How can you only use the mass of the car and not include the driver?  I do understand why the force on the driver only includes the mass of the driver.  The seat exerts the force on the driver's mass.  But that seat force would also be backward on the car, and so to accelerate the car at the acceleration determined would require the additional force on the driver, correct?

The net force on the car is its mass times its acceleration. Sure, the driver may be pushing backward on the car; the road is also pushing forward on the car.  That's all true, but all the question asks for is the net force on the car, which does mean the force of the road minus the force of the driver.  I don't know the value of either of those forces.  I just know about the car's mass and its acceleration.

Similarly, you're right that the only horizontal force on the driver is the force of the car.  That's the net force on the driver.  Yet, that net force is still equal to the driver's mass times her acceleration.

Since the car and driver move together - when one speeds up, the other does too, by the same amount - they have the same acceleration.  Thus ma must be bigger for the more massive car.