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14 July 2018

2016 AP Physics 1 problem 5 waves on a vertical string - video from Walter Keeley

In my PWISTA* AP Summer Institute, participant Walter Keeley tried to set up the waves-on-a-vertical-string problem from the 2016 P1 exam.  (Click the link, and go to problem 5.)  The problem asks students to explain, in a paragraph, how a picture of standing waves on a vertical string provides experimental evidence that the wave speed depends directly on the tension in the string.

*Putnam-Westchester Industry & Science Teacher Alliance

The correct approach:  The picture shows a longer wavelength in the standing wave near the top, where the tension in the significantly-massive string is greater.  Since the frequency of vibration is provided by the wave generator and is the same throughout the string, by v=λf the larger wavelength means larger wave speed.

Walter initially had no more success setting this up than I had over the years.  The strings I've used have simply not been heavy enough.  Walter tried a seriously massive rope, but it wasn't flexible enough to show a clear pattern.

It was Lorren, one of our participants, who suggested using ball-chain to provide both mass and flexibility.  It was Tom who pointed out that you'd have more luck seeing the wave if the generator didn't merely vibrate left-and-right, but in a small circle.

So, when Walter had some downtime at robotics camp, he obtained some chain (from True Value Hardware at 40 cents per foot).  Walter had to build his own geared motor to vibrate the chain:
Photo credit Walter Keeley

Walter used his phone to create a video of the vibrating chain; the app Hudl Technique is what he (and my students, too) use for playback, because it does slow motion and precise timing.

And oh my goodness, look at Walter's video: 

Look how you can see -- and measure, if you want -- the wavelength change.  Walter even arranged - presumably by trial and error with the frequency selection - to have the same three-and-a-half antinodes that are shown in the original AP problem.

Do you have a beautiful experimental setup of a released AP Physics problem?  Send pictures, and I'll be happy to post it on the blog with credit to you.  If you've never set up released AP problems in your lab, well, that's an awesome April open-ended lab assignment.

04 July 2018

2017 AP Physics 1 problem 1 with the draining batteries: we set it up

On the last day of my AP Physics 1 summer institutes, participants are asked to pick a released exam question, solve it... and then set it up experimentally.  We always get some simple yet elegant setups, like Rebecca’s modified-Atwood-turned-projectile from the 1998 exam; some complicated setups that don’t quite work but point in the right direction, like Lorren’s attempt at the 2018 collision-on-a-horizontal-spring problem which got us thinking about vertical springs and velcro; and some complex and elegant creations that go beyond just the AP question, like Quinn’s measurement of the kinetic friction coefficient that can then be used to determine the mass of an unknown block.

Several free response problems over the last few years have presented significant experimental challenges, though.  When the bumpy track problem came out on the 2016 exam I heard from so many teachers, "this isn't real, no one would actually set that up.  Well, we did last summer. 

This year one participant, Frank, took on an even greater challenge: the paragraph question about draining batteries, from the 2017 AP Physics exam.  It postulates connecting light bulbs to a battery in three ways, and asks which will drain the battery soonest.  

Which battery drains first depends on the power dissipated by the circuit: since power is energy per time, the shortest time to drain the battery will come from the circuit dissipating the largest power. By V2/R with the battery always providing the same voltage, whichever circuit has the smallest equivalent resistance will dissipate the most power.  So the bulbs in parallel will drain the battery first, followed by the single bulb, followed by the two bulbs in series.  That's the theoretical solution.

Frank and I and our lab assistant Tom brainstormed three approaches to this experiment.

(1) We could finesse the problem by simply observing brightness, which correlates directly to power.  That's almost trivial - we connected three sets of bulbs to three identical batteries, and we saw the parallel bulbs brightest, the single bulb next brightest, and the series bulbs dimmest.  Great for a simple demo, but this sort of thing has been done before.    

(2) We could get three fresh batteries, hook them up, and set up an iphone to do a time lapse video.  I don't know offhand how long it would take to drain fresh AA batteries, but it's not gonna happen within a single lab period; and that's all we had at our institute.  I do hope that some reader will set up the time lapse and send it along.

(3) Tom noted that he had some "big blue" capacitors available.  These are 27,000 microfarads and can operate up to 25 V.  Though the study of capacitors is beyond the scope of AP Physics 1, it's not hard to explain to students that these things store a fixed amount of energy (for a given starting voltage), and then that energy dissipates rapidly.  

How rapidly?  Your students don't need to know, but the time constant of discharge depends on the product of the equivalent resistance and the capacitance.  We want an equivalent resistance in the dozens of ohms to get time constants on the order of seconds.  Frank and Tom found some Christmas lights; when these were connected via alligator clips, the dimming of the lights over a few seconds as the capacitor drained was easily visible.

Which drained first?  It was sorta hard to tell with the naked eye.  So we clothed our eyes with slow motion video on our phones.  

Frank charged the capacitors.  He connected a switch that, when thrown, would simultaneously connect each circuit to its capacitor.  Here's the result:

Just like we predicted - physics works.

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 Charles Lee 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.

16 April 2018

Students: you don't need more AP practice problems.

This is the time of year when physics teachers tend to be approached by hyperconcerned AP students worried about the upcoming exam.  "Can you find me more practice problems?  I want to do as many as possible so I'm prepared." 

The teacher's answer should almost always be "no." 

Huh?  Um, Greg, why not provide more practice problems?  Why shouldn't students be encouraged to study more, especially when it's they who are taking the initiative? 

I'll give you two reasons.

For one: It's the quality of preparation that is useful, not the quantity.  I've assigned an enormous number of AP practice problems as homework, tests, quizzes, and in-class exercises.  For each of these, I've helped students understand what they've done right and wrong, and how to do better on the next exercise.  We don't just do a problem and forget about it. I check homework and make students redo problems that they substantially didn't understand.  We grade each others' quizzes in class.  We do test corrections.  In-class exercises lead to discussion, or sometimes experimental verification.  

In contrast, handing students a thick pack of review problems encourages the mindset of just getting the answer right, without the deep engagement required when working in conjunction with a formal class.

And a thick pack of review problems can't possibly be useful unless the problems are themselves high quality.  I just got an email from a prominent supplier of inferior physics lab equipment saying something like "With the AP exam approaching, try these practice questions for $99!"  Stupid, stupid, stupid.

Authentic old AP free response questions with authentic rubrics are available to students through the College Board's official site.  The five steps book has an enormous number of questions, each with a thorough solution, not just an answer.  These are publicly available to students - and the 5 Steps book costs a fraction of $99.

I've seen some of the material that's peddled by others as AP exam preparation.  It's uniformly terrible.  Pick up a random other prep book and look at the content and style of questions.  The GOOD ones are repackaged AP Physics B material, heavy on the calculation, nowhere near the style and depth of the true AP Physics 1 exam.  Folks, there aren't that many people in the country qualified to write AP Physics 1 questions.  The vast majority of those are already in the employ of ETS and the College Board.

For two: I refuse to feed the test anxiety beast.

At this point in the school year, with three weeks to go before the AP exam, I'm tapering my AP class.  We are winding down, not up, in our preparation.  I have a lot of reasons for relaxing my demands in April, but a primary one is to emphasize with my actions that "test anxiety" can be limited by building a positive culture.

We've done the necessary deep practice all throughout the year.  I've ensured every day that students have not just done the homework, but they've paid careful attention to it; if they don't, they come in for consultation.  For each incorrect answer on a test, my students write a clear correction.  We've built the habit that practice doesn't make perfect; perfect practice makes perfect.  

By year's end, all of my students know what they can and can't do.  They understand their strengths and weaknesses, and how to play to those strengths on the AP exam (because they've done so on eight AP-style practice tests already).  They know how to handle the adversity that is sure to come on an exam on which 70% is a top score.  They've already done their practice problems on tests and quizzes and homework - that's how they know all of the above.

What message would I be sending, then, if I kept handing ever-larger sets of practice questions in the lead-up to the exam?  I'd be destroying the confidence I've struggled so hard to build.  There's no need to scare students into studying more.

Top students tend to work themselves into a state right before an exam.  Or, they do more and more practice problems as a way to show off to their peers... it's the secular equivalent of the holier-than-thou churchgoer.  But we're talking about top students!  They don't need to study more.  What are they going to do, turn their 5 into a 6?  Make them relax, both for their own mental well-being and for the sake of their classmates.

But what about students who will struggle to get a 3?  They will do better with a low-key approach, too.  Rather than shame them for not knowing everything - which is what we do, like it or not, when we shove a stack of practice problems at them - focus these folks on just a few topics that they can improve upon.  They're not going to get a 5 because they studied for days.  But, a few judicious hours here and there might well secure that 3.  

So, don't encourage more practice.  Encourage good, targeted practice in the weeks before the exam.  Encourage a relaxed, confident attitude in the days before the exam.  When students recognize intellectually and emotionally that AP exam day is just another day at the office, then your class is ready to rock.  No extra practice questions required.

02 April 2018

Targeted quiz to check for homework understanding

In the previous post, I discussed an AP Physics 1-style problem about Coulomb's law. That was my students' homework assignment due Monday.

On homework, students are allowed and encouraged to collaborate while obeying the five-foot rule. This means they may talk to one another, even look at each others' work, as long as they separate themselves whenever they're writing something to be turned in.

Yes, I do suggest you ask for obedience to the five-foot rule, even if you're at a school where copying answers is an unfortunate part of the culture.  It's difficult politically to enforce a "no copying" rule, partly because of the feigned innocence and ruthless mother-bear-defending-her-cub response you'll get by even suggesting that students might have engaged in cheating.  It's not difficult at all, though, to show that the five-foot rule has been violated, and to demand adherence: since you're not framing any requirements in terms of integrity or honesty, you tend to get compliance from students and support from parents and administrators.

Nevertheless.  I hear from teachers all the time that their students simply copy the best student's answer on homework problems, rendering meaningless the whole activity of doing homework.  Even at my current school where the students are impeccably honest and careful about the five-foot rule, I still have a minority of the class who write down a solution without thoroughly understanding it.  That's not useful to anyone. How is it possible to make students engage with the assignments?

One approach that's been successful for me is the targeted quiz.  I collect the homework... then I assign a five minute quiz asking questions similar to what was on the homework.  The idea is, students who understood the problem solving process on the homework will do well on the quiz; those who merely mimicked a friend will not do well, and thus the quiz will provide the context they need to figure out what they didn't get.

Below are the questions I asked based on Monday's Coulomb's law problem.  Take a look... see how a student who understood the homework, even with assistance from a friend, will answer quickly and confidently.  See how a student who copied a friend's answers will be clueless, especially on 1 and 3a.

That's what makes a good targeted quiz - it sends the message that answers don't matter, comprehension of solution methods does matter.  And I don't have to lecture or nag at all.  I just go over the quiz.

1. Show how to find the distance between charge 1 and charge 2 in terms of X and θ.

2. Draw and label vectors to represent the forces on object 1.

In part (c), you were asked: Using the conditions of equilibrium, write—but do not solve—two equations that could, together, be solved for θ.

3a. Explain in one sentence how we are going to get the two equations.  

3b. If we are writing two equations, how many unknowns may there be in the equations?

3c. What is incomplete about these two equations as the solution:  

Tcosθ  = mg
Tsinθ  = Fe

31 March 2018

Coulomb's Law problem for AP Physics 1

When you're looking for AP Physics 1 problem ideas, start with old AP Physics B exams, especially the B exams between 1996 and 2014.  These are a treasure of interesting situations, many of which can provoke more AP Physics 1 style questions than you can possibly ask in one school year.

In AP Physics 1, Coulomb's Law problems are restricted to the force exerted between two isolated charges.  Usually Coulomb's Law questions will appear on the multiple choice.  You'll see semi-quantitative questions, like "double the mass and halve the charge of one object, what happens to the electrical force?"  You'll also see questions about the "relationship" between Coulomb's law and Newton's Law of Gravitation.  

What about the free response? Interesting problems like 2010 AP B #3 are only appropriate for AP Physics 2 - that problem includes three charges, and asks about the vector sum of several forces.

Take a look, though, at 2009 AP B #2.  There, two equal-length strings support objects of known mass and charge.  The strings each hang an a known angle from the vertical.  Ooh... just two charges, but the situation is ripe for questions that go beyond straight-up plugging into Coulomb's Law.

This problem isn't suited as-is for Physics 1, of course.  It asks about electric field lines, which are not part of P1 (and are deprecated in P2 in favor of vector field notation).  It asks about electric potential produced by several point charges.  Skip those parts.

To rewrite for Physics 1, start with parts (c) and (d) of the original.  Part (c) asked for a free body diagram of one of the two hanging objects.  Part (d) brilliantly asked students to write two equations that could be solved simultaneously for the tension in and angle of the string.  Explicitly, students were instructed NOT to solve the equations!  Since AP Physics is not a math class, we don't care whether students can carry through the algorithmic solution to such a problem.*  We do in fact care whether students can set up a system of equations, and then recognize whether the system is solvable.

* More precisely, we already know that our class of 20 students can create 20 different solutions.

So I assigned parts (c) and (d) only, and added a third part

While the charge on object 1 remains positive Q, the amount of positive charge on object 2 is increased to 2Q.  Describe any changes in the arrangement, and explain the physical reason for those changes. 

I don't need the precise location of the new equilibrium position - that's way, way beyond the scope of AP Physics 1.  All I'm looking to see here is indication that the separation between charges will increase, because the force of object 2 on object 1 (and vice-versa) is now larger.  At the original locations, the horizontal component of the string's tension would no longer balance the electric force, and so there'd be an acceleration outward.

Another possible question based on this situation might be to ask whether, with charges Q and 2Q, the angles of the two strings would be the same as each other, or different from each other.  

You got another question based on this situation?  Add it in the comments!

15 March 2018

Thoughts about mentoring new teachers

I was asked, "what does your department do to mentor new teachers?"

We haven't done anything formal, nor do we really want to.  The veterans have generally taken it on themselves to keep in touch with the new people, answering questions and being available as we can.  My philosophy is, there's no point in talking at new teachers before school starts more than we already do (and we already talk at them way too much).  Teaching – for the first time, or at a new school – must be experienced.  "Mentoring" consists of showing through our actions the type of teacher we hope that the new folks will become.  The best way for new young teachers to learn is to watch the pros, and then to talk shop.  

I think veteran teachers underestimate the fear of failure and the desperate, beaten-down mindset of the new teacher.  (Possibly because we have consigned those dark years to the deepest inaccessible recesses of our brains.)  I remember being so, so worried about doing things wrong, knowing that I was not as comfortable and in charge as my colleagues, knowing that there were complaints everywhere from students testing my resolve, feeling the disapproval of my colleagues.  I beat myself up so much -- and colleagues and bosses beat me up so much -- that I never really realized what a damned good job I did in my first couple of years.  I felt like I was expected to be perfect, and then to get better; I felt like, even though I recognized and tried to learn from my mistakes, that nevertheless I was always in danger of being sacked for my lack of perfection.  (And in fact my contract was not renewed.  At two separate schools.*)  It wasn't until I had been teaching for nearly a decade that I stopped feeling like I had to prove myself every day.

* How’d you like your physics programs now, schools that booted me out the door?  

My wife taught English as a 22 year old who was the youngest person on faculty, and one of only four women.  She similarly felt the constant disapproval from colleagues, the *perceived* disapproval from the older men in her department even when it wasn't there, and the disapproval from herself for not being practically perfect in every way.  She never noticed just how badly she was being treated by the school; she just assumed anything that wasn't perfect was all her fault.  Her colleagues, her students, and the administration drove her out of teaching.  And that is too bad, because she was really, really friggin' good for a new teacher.  She would be bloody amazing now had she stuck with it.  

Those same effects were *this* close to driving me out of teaching, but for one kind offhand comment from a veteran.  I'm glad I stayed, but I still sometimes wonder why I did.   

I suppose I think of mentoring new teachers much like sabermetrics thinks of evaluating baseball managers -- it's not possible for a manager to significantly improve his team's chances of winning by making "good" strategic choices.  The best managers statistically are the ones who back off and don't make actively stupid strategic decisions.  In that vein, formal observations and formal, scheduled discussions are often more intimidating / loaded with perceived disapproval than effective.  But those veterans who take care to develop personal and professional relationships with new teachers will place themselves in a position to serve as a sounding board, and then to gently offer advice where asked.

I can tell you what sort of "mentoring" did NOT work well.  It was NOT effective when I was told to use the calculus teacher down the hall as a resource -- he was actively unfriendly, unhelpful in a practical sense, and unenthusiastic about helping; he threw in some piss poor advice to boot.  I needed someone to tell me when I did things well, to talk through my ideas without dismissing them immediately out of hand, to allow me to try new things without prejudging the results.  I needed someone to vigorously shut down ridiculous complaints, to smite loser arseholes (like those who were caught but not held accountable for hollering "BITCH!" outside my wife's classroom her first year), to quelch the malicious gossip from students and less-than-friendly colleagues before it could snowball.  Then, only then, did I need someone to give me advice.  What I actually got was a bunch of old folks who freely dispensed advice, but who also sent messages in body language that I wasn't part of their club, and that it was largely my fault that the students whined about me -- blame the victim.  Know that this isn't just a me and my first school issue -- my wife had the EXACT SAME experience.  And I'll bet you know of someone else with this experience, too.

Just telling a new teacher to work with veterans who aren't invested in mentoring does not work.  One cannot be mentored when every event, every question is loaded with judgmental baggage.  It takes a special, special veteran teacher to build enough trust with a new young teacher such that the new teacher is willing to open up, to ask tough questions, to ask for advice, and to take advice without feeling sandbagged.  The number of veteran teachers capable of building that trust is, sadly, negligible. 

We should, wherever possible, put new faculty in close physical proximity with veterans who are easy to talk to.  A while back we got three physics teachers new to my school, two of whom were new to teaching as well.  Having the three of them share an office was priceless.  I was just down the hall, and was always in the coffee room.  Just that proximity let us develop a camaraderie.  We want new folks nearby to those who will be both helpful and nonjudgmental; folks who will not butt in unless asked (or unless it's blindingly obvious that butting needs to be done).  Even if no actual advice is dispensed, just watching the professionalism of our veterans builds a positive corporate culture in which we have to convince faculty to STOP working rather than to start.

Point is, there can be no formula for mentoring new or young teachers.  So much is driven by personalities and relationships.   I'd say that former football coach Clint Alexander has been a better academic mentor to me, and to a large number of our faculty, than pretty much anyone else I've met professionally - when I had trouble with a student or a colleague, I asked Clint for advice, even though he wasn't a fellow teacher in my department.  Our head track coach has mentored his assistants in a way that goes well beyond track.   We can't know ahead of time whose spouses or families will get along with those of a new teacher.  We can't predict who will get along well within a department, or who will become more friendly with teachers on a coaching staff.  

All we can do is continue to hire and develop veteran teachers who remember what it was like to be new, and who consider it their job to support new folk in an atmosphere of authentic, non-judgmental caring.  Keep the control freaks, those who judge, those who give too much credence to silly kid complaints, far away from the newbies.  Keep those who work hard and professionally visible and prominent; make it clear to the new teachers through our actions what phenotypes of teachers are valued by the school.  That's all we can do.  After that, teachers will have to sink or swim on their own abilities.

07 March 2018

Handing back work

It seems like such a small thing... but the manner in which you hand back routine student work makes a difference, especially in the tone of the class.

Firstly, let me kindly point out the underlying principle that, speaking in generalities, students don't care about a routine assignment once they've handed it in. 

Don't believe me?  Try an experiment.  Don't place students' work directly on their desks.  Rather, place the work easily accessible in the back of the room, or on top of a table down the hall, somewhere that requires a student to exert extra but minimal effort to get the assignment.  Do this for at least three routine assignments. By the third, I'll bet that only a couple of students bother to go get their papers, even if "going" to get the papers requires merely a walk to the back of the classroom while people are filing into the room.

For years now I've handed back student work in a slotted cabinet in the back of the room.  Each student has a numbered slot into which his papers are placed.  Only one or two students go back there on any given day, unless I make them. 

It's important that students know you're somehow looking at and evaluating their work; otherwise they won't take it seriously.  In the long term, your students really do appreciate your care in crafting, reading, and handing back routine assignments, because they will eventually recognize that your care for their assignments is an expression of your care for them personally.  Right now, though, yesterday's problem set might as well be as ancient and relevant as the OJ Simpson trial.

So why don't I force students to look at their previous work by placing it on their desks?  

In that case, I'm practically begging students to argue about points, to wonder why they only got two out of three when their friend got three out of three "and he said the same thing!"  I'm encouraging questions on the order of "well, if you really think about it, this answer could be right, can I have some points back?"  Faced with a graded paper, my students look straight at the final score, then at their classmates' final scores; then they start rationalizations that go so far beyond sour grapes as to become aged wine.

By returning papers in the back of the room, you have a grace period to tell the class briefly "hey, on last night's problem, you can't set the tension in the rope equal to mg, because the object is speeding up.  You have to write Newton's second law for both objects and combine the equations."  Everyone paid attention to that 20 second statement; everyone is now constructively considering whether they made that error or not.  Had their papers been in front of them, 3/4 of the class would have been leafing through the pages trying to mine for points, and won't have heard your statement anyway; half of the rest are considering whether the number of points they lost for that error was commensurate with their sense of justice, or whether they should summon Batman to fight for their points back.

Usually, a student who makes a couple of errors on a problem set doesn't need to have those rubbed in his face; it's far better just to mention common errors in general, but then move along.  They've been brought up in a system where 93% is an A, where anything less is on par morally with trigamy.  They get way too upset about their lack of perfection.  If they don't see their paper, even if they're purposely ignoring their paper in order to avoid confronting their imperfection, then that's a positive result.

Of course, there are times when you need a student to take a look at a routine assignment, particularly when that student's responses were nowhere close to on target.  In that case, require an extra help session of that student, and make him go get his work to show you.  This is the time to make the student redo the problem the right way... when there's no social value to the performance art inherent in "but teacher, can't I have pity points for writing F=ma?"  Redoing the problems from scratch can build confidence and bust misconceptions.

And, handing back the rare major test can be done without recourse to the back-of-the-room method.  I suggest either handing back a blank test with an indication of which problems require correction; or, discussing common issues briefly while holding on to the tests in the front of the class.

For regular assignments, though, you avoid a lot of headaches by making students take a bit of an extra effort to fetch their work.  You can focus on physics, rather than lawyerly discussions about grading.

24 February 2018

Mail Time: Is Pluto's Angular Momentum About the Sun Conserved?

Dear Greg, 

I have been reviewing with my students and want to pose this question to them but am having a bit of difficulty with the solution:

The dwarf planet Pluto goes around the sun in an elliptical orbit.  Consider Pluto only.  Is its angular momentum about the sun conserved?  Justify your answer.  

I know that in an elliptical orbit the distance between the sun and Pluto would change and therefore angular momentum would not be conserved.  However what is throwing me off is considering Pluto only.  Does this mean you do not take into consideration the elliptical path?

Angular momentum is conserved when the system experiences no net torque.  Pluto alone is the system.  What forces act on Pluto?  Just the (gravitational) force of the sun.  Does that force provide a torque about the sun?  No - the force of the sun on Pluto is always directed toward the sun, so there is no lever arm for that force about the sun.  (Another way to say it: the line of the force of the sun on Pluto goes through the axis of rotation, which is the sun itself.)  So angular momentum is conserved.

How does that square with the elliptical orbit?  Angular momentum for the point-object Pluto is mvr, and that can’t change.  So when the distance from the sun r is small, the speed v is large, and vice versa.  That is in fact the case for all planets.  The earth moves faster around the sun in (northern hemisphere) winter, when we’re about 3% closer to the sun, than in summer when we’re a wee bit farther away.


10 February 2018

Oh My Gawd, It's a Test!

How does your class react when you announce an upcoming test?

Ideally, they say nothing.  They register the reminder with the same demeanor with which the New England Patriots took the field for their seventh Super Bowl this century: calm confidence mixed with a tinge of nervous anticipation.

Too often, though, your announcement incites a game of misery poker, each student in turn offering a complaint, a sarcastic comment, or an increasingly dramatic vision of how the upcoming test will ruin his life.  How do we as physics teachers encourage an appropriate culture around testing?

It starts with the very first comment about the very first test.  If you let small passive-aggressive comments go unchallenged early, they'll eventually turn into big actual-aggressive comments that can't be mitigated.

I deal firmly, kindly, and somewhat publicly with the student who fans the flames of drahma.  "Oh my goodness, I studied for hours and I know I'm gonna fail.  Here goes nothing."

In front of all, I'll put the same phrase in the context of sports: "Johnny, you're a baseball player... you just said to your team and coach, 'I'm next up to bat.  Just know that I suffered through these horrible practices all week, I'm still terrible, and I'm gonna strike out right now before I let a grounder go through my legs next inning.'  What would your coach say?  Oh, that's right, she'd bench you.  She'd replace you with someone who wasn't explicitly and aggressively saying he'd let the team down."

On a team, such chicken little talk gets the social shunning it deserves.  Why do we let it pass in academics?  Nip it in the bud.  The silent majority of students will appreciate the more positive atmosphere you create by shutting down the drahmatists.

If a student continues to kvetch, or even if he gives me negative body language, I'll take him aside and appeal to his* ego.  "So, Johnny, you're one of the better students in the class.  How do you think your words make Joey feel?  He's going to think, jeez, if JOHNNY thinks he's gonna fail, what chance to I have?  The class needs positive leadership from you, Johnny, and leadership begins with poise and confidence."

* I teach at a boys' school.  I imagine that my approach would work similarly in a co-ed environment, but I have no direct evidence.

But students have legitimate questions about the upcoming test.  Of course.  I can't shut those questions down... I must communicate the form, content, and performance expectations of the test.

Nevertheless, I don't need to answer silly or irrelevant questions; I don't need to answer questions twice; and I don't need to answer passive-aggressive questions that are really whiny complaints.

What's going to be on the test?  Answer it once per year: everything we've discussed.  [Smile.]  I'm not doing my job as a teacher if I give you permission to forget everything I've taught you.

Make the format consistent and transparent.  Hand out the cover sheet ahead of time, indicating the number of each type of question and time limits.  If the students don't expect surprises; and better, if the gossip amongst generations of students never includes stories of surprise or gotcha questions; then you can more reasonably demand that your students stop with the fear-mongering.

Will there be a curve?  Again, answer out loud once per year: the cover sheet includes the point values for each section, along with the number of points necessary for each grade.  If you pass a sheet like this out for every test, there's no reason for anyone to ask about it in class.

What if I fail?  Are there retakes?  Can I do extra credit?  Can I lawyer up after the test to convince you to give me an A?  Can we go back to that fourth down play when New England didn't cover Nick Foles and the Eagles scored the winning touchdown?  What do you think, should we give the Patriots another try?  I mean, they've worked their tails off all season, they tried so hard, can't we have some mercy on them?  

I have a connection with most of my class through sports.  Feel free to use other avenues of public life.  "Can we go back to early November 2016?  Remember when Ms. Clinton didn't campaign in Wisconsin, Michigan, or Florida?  Perhaps the Republican party would allow a re-vote, or some extra credit for Clinton in the electoral college... after all, she tried so hard..."  

Whatever works for you and your class.  Just shut down the complaining.  It will be appreciated by most, and worth it come exam time.

05 February 2018

Carry on

Welcome back, class.  I know the first day of school after break is sorta useless, and I know it's hard to remember things we talked about two weeks ago, so... you just lost a day of teaching.

Look, it's not like I'm blind or stupid.  I notice the days that are more difficult to maintain student focus.  Typical culprits include days immediately before or after a scheduled vacation or a major non-academic event like the state championship football game or the prom.  I know my seniors will engage far better in the fall than the spring, while freshmen are the opposite.

Fact is, these difficult days are still school days.  I've still got a job to do; the AP exam or the class final doesn't get pushed back because of last night's Duke vs. North Carolina game.  These days will never be as effective as an ideal day.

But that doesn't mean simply punt on them.  Have a plan.  Do something as active and engaging as you can manage.  These aren't the days for long discussion or lecture sessions, not for testing, not for difficult creative lab work.  These are good days, however, for straightforward, active lab work.  For one of those The Physics Classroom interactives.  For starting a new topic with an eye-catching demo showing a discrepant event.

No matter what your plan, though, your demeanor is the most critical component to the quality of your class on a difficult school day.  

Why do students consider that, for example, they shouldn't have to think too hard in class the day after the Super Bowl?  Because all the adults around them say so.  (Not, in the vast majority of cases, because the students were out drinking and climbing greased lampposts until 5:00 am the night before.  Philadelphia-area schools possibly excepted.)

If you start class with a pre-made excuse to not pay attention, well, why are you surprised or disappointed when the students don't pay attention? 

Keep calm and carry on.  "Did you see that game last night, Mr. Lipshutz?!?"  "Yes, it was fantastic!  I'd love to talk through the Eagles' gutsy playcalling at the lunch tables - amazing.  For now, though, here's our three minute bell quiz which will remind us of last week's topics... you may begin."

30 January 2018

US Invitational Young Physicists Tournament 2018 results and 11-year participation

A big thank you to Randolph College in Lynchburg, Virginia for hosting the 11th annual US Invitational Young Physicists Tournament.  This past Saturday and Sunday, a record fifteen teams competed in physics fights over four problems involving:

     * measurement of the moon's orbit
     * electromagnetically coupled oscillators
     * blackbody radiation laws applied to light bulbs
     * projectile motion in air

This year's winners, in their third visit to the tournament: Phillips Exeter Academy of New Hampshire, led by physics teacher Scott Saltman.

In second place was The Harker School of California, led by Mark Brada and Miriam Allersma.

The winner of the Swartz Poster Session was Shenzhen Middle School, led by Chen Shaorui.

The overall order of finish is below.  Our rules state that a number of places are shared by similar teams.  The ** means that this team won the prestigious Bibilashvili Award for outstanding physics.  It is awarded to teams with superior physics understanding, irrespective of their placement, at the tournament director's discretion.

     Phillips Exeter Academy**

Second place:
     The Harker School**

Third place:
     Rye Country Day School**
     Cary Academy**
     Yorba Linda High School**

Fourth Place:
     High School affiliated with Renmin University, China

Fifth Place:
     Shenzhen Middle School**

Sixth Place with Bibilashvili Medal:
     Woodberry Forest School**
     Nueva School**

Sixth Place:
     Pioneer School of Ariana
     Qingdao No.2 High School
     Vanke Meisha Academy
     Princeton International School of Science & Mathematics
     Spartanburg Day School
     York Country Day School

And scroll down to find the list of all teams who have participated in the USIYPT since its inception in 2007.

Next year's tournament will be January 26-27 at Rye Country Day School in New York.  If you'd like to come as a juror - or if you'd like to bring a team from your school - please email me.  More information about our tournament is available on the official website,

Participating USIYPT schools in the 11 tournaments since 2007:
The Harker School, California – 9 tournaments, 3 championships
Rye Country Day School, New York – 11 tournaments, 2 championships
Shenzhen Middle School, China – 7 tournaments, 2 championships
Woodberry Forest School, Virginia – 11 tournaments, 1 championship
Phillips Exeter Academy, New Hampshire – 3 tournaments, 1 championship
Brisbane Girls Grammar School, Australia – 2 tournaments, 1 championship
Raffles Institution, Singapore – 1 tournament, 1 championship
Pioneer School of Ariana, Tunisia – 6 tournaments
Nanjing Foreign Language School, China – 5 tournaments
Princeton International School of Science and Mathematics, New Jersey – 4 tournaments
High School affiliated with Renmin University, China – 4 tournaments
Cary Academy, North Carolina – 3 tournaments
Phoenixville Area High School, Pennsylvania – 3 tournaments
Wildwood School, California – 3 tournaments
Oak Ridge High School, Tennessee – 2 tournaments
North Carolina School of Science and Mathematics– 2 tournaments
Oregon Episcopal School – 2 tournaments
Vistamar School, California – 2 tournaments
Guilderland High School, New York – 2 tournaments
Qingdao No. 2 High School, China – 2 tournaments
Vanke Meisha Academy, China – 2 tournaments
High School of Jur Hronec, Slovak Republic – 1 tournament
Calverton School, Maryland – 1 tournament
Madeira School, Virginia – 1 tournament
Pioneer School of Manzeh 8, Tunisia – 1 tournament
Georgian English-Spanish School, Tbilisi – 1 tournament

Participants, team leaders, and jurors at the 2018 USIYPT at Randolph College

20 January 2018

Guest Post: Bill Payne on using Audacity as a timer

Folks, Bill Payne submitted a note to the Modeling listserv that'd I'm sharing with permission as a guest post.  I always love using free software to get data that otherwise would require expensive commercial apparatus; Bill has a fantastic method of determining time intervals to millisecond precision using audacity - google "audacity" to get the free download.  Bill also provided a link to a set of follow-up questions in this excel file.  Take it away, Bill:

Bill: My kids and their teacher love shooting darts, both the kind with the suction cup end and the round end.  Last week we used the computer program Audacity to time how long it took a dart to reach a target 1 m, 2 m and 3 m away.  Audacity will give the time in milliseconds between any two sounds: the firing of a dart gun and the dart hitting the target, or you can roll a steel ball down a ramp, see where it hits the table, and place a whiteboard a measured distance from the point the ball hit the table.  Audacity is much easier to set up than photogates, and it's a free download.

Audacity will let you highlight and delete the parts of the graph you don't need, such as the flat line leading up to the firing of a dart gun.  That sets the time of the firing of the gun to zero.  Then expand the graph with Command 1 to stretch it out (repeat Command 1 to stretch it out more and more).  Cut the rest of the leader out to get the firing right on zero.  Then put your cursor on the beginning of the part of the graph when the dart hits its target, and read the elapsed time at the bottom in milliseconds.  Calculate velocity: V = d/t.

Only problem is, now the kids want to have a dart-gun war.  I put them off to the end of school.  :-)

Bill Payne
Physics Teacher
Host of modeling workshops at Birmingham-Southern College

16 January 2018

Trust the Process

I had a rough class Tuesday.

I thought about changing course.  I didn't - I kept plunging forward.  And everything worked out beautifully.

We were working on the direction of force and motion using these in-class lab exercises.   I encourage you to take a look.  They involve three situations:

1. Jumping on or off a force plate
2. A hanging object attached to a cart
3. A fan cart attached to a hanging mass over a pulley

In each case, two forces act on an object.  Students are asked to determine the direction of acceleration, and then which of the two forces is larger.  Finally, they go to the back of the room where I have equipment set up.  They use force probes and plates to verify their predictions.  Nothing here is quantitative - we don't predict a value for the tension in a rope.  In this exercise, we only are comparing which of two forces is larger.

Sounds easy enough, right?  "The object moves up and slows down, so acceleration is downward.  Net force is also downward.  That means down forces are bigger than up forces, so the weight is bigger than the tension in the rope."

Hah.  No, on Tuesday I kept hearing "In order for the object to move upward there must be more forces pulling than pushing upward than downward.  And the object has weight, so the tension in the rope is massive.  Plus here are three more sentences full of nonsense please count it right."  I'm barely making this up.  My class was getting palpably frustrated.

In 80 minutes of lab work, about half the class completed one exercise; the other half completed zero.  I had a major assessment scheduled for the last part of Thursday's 90 minute class.  Would the students be ready?  Should I reschedule?  Should I stop the lab work and start just doing problems in front of the class to assuage their frustration?

It's hardly ever a good idea to slow the pace of the class just because students seem to struggle the first time seeing a difficult concept.  If Newton's second law were easy, I wouldn't be employed. Doing problems in front of the class doesn't help anyone - the only way to learn physics is to make mistakes, then to learn from those mistakes.  Students must be active, not passive, otherwise they'll make the same mistakes on the assessment that they were making in class.

So I pushed on.  Thursday's class began with a brief quiz, followed by five minutes of discussion about the problem set.  Then back to lab work for 45 minutes before the major assessment.  I braced myself...

The pace of work ascended to the next available energy level.  Everyone finished at least two exercises.  About half the class finished three.  And scores on the major assessment were as high or higher than ever.

Trust the process.  When you're doing creative lab work, or any sort of physics teaching that isn't just you telling students how to do problems, frustration and wrong answers are a natural part of the learning process.  Let that frustration happen.  Keep morale up as best you can.  Because the epiphany will come.

And what of the two or three students who didn't perform well on the assessment?  Wouldn't they have been better served by a different approach?  Perhaps, but not likely.  In any case, their epiphany will come, too.  In fact, during the next Monday's test corrections class, one of these three poor performers looked at me with a wry smile.  He said, "you know, I kept making dumb mistakes.  I should have known these answers, they seem really easy now.  I'll get these next time no problem."