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## 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."

## 29 December 2017

Dear Mr. Jacobs,

Hope you are having a happy holiday. We just got Johnny's grades. While we are pleased that he tries so hard and cares so much about your class, Bunny and I are disappointed that his exam didn't push his grade up from a B+ to an A-. Junior year is especially important for college, and Johnny's goal is a 3.6.  We are hoping you will reevaluate some of the more subjective portions of the course grade, or perhaps allow him to do some extra credit to bring him up to an A-.  We know you want the best for those you teach, and you wouldn't want to see a diligent student denied his future because of just a few points here and there.  When is a good time for you to meet with us to discuss his grade?  Sincerely, Mr. Smith.

No, reader, I didn't hack your email account - this is just an amalgam of all the similar notes and conversations I and my colleagues have dealt with over the years.

How to respond?

Not with aggressive complaints, not with scathing wit and sick burns.  Yes, Mr. Smith's note is full of passive-aggressive assumptions of bad faith.  So what.  Bring it down, bring it down.*  You are in charge here.

* Han Solo, in The Force Awakens, as Finn delights too much in his dominance over Captain Phasma.

Part of what gets your dander up as a teacher in this situation is fear.  Fear that you'll be trapped in repeated unproductive meetings defending your grading procedures to ever higher-level and physics-clueless administrators. Fear that Johnny will begin spreading discontent and despondence throughout his class.  Fear that if you don't emphatically shut down Mr. and Mrs. Smith then Mr. and Mrs. Jones will start canoodling to change Brittany's grade, too.

Yes, all these things could happen.  But the fact remains - you are in charge. You assigned the grade; a nasty email from a parent puts you under no obligation to reevaluate the grade.  A batter may give the umpire a Look and the manager may scream "where the \$&*# was that pitch?"... yet the umpire should not timidly change the call from "strike" to "ball."

However, neither should the umpire get sucked into a heated discussion of his or her strike-calling philosophy.

Let's calmly consider the most politically practical and appropriate response to the Smiths - the response that's least likely to cause all the teacher's fears to be realized.

Dear Mr. Smith (copy to science chairperson, principal, and college counseling director),

Thank you for your note.  Johnny has indeed been a diligent, positive contributor to our class this semester. I'm pleased with how hard he's worked at developing his physics skills. The semester grades are final.  I'm confident that Johnny can continue to perform well in the second semester.

Have a wonderful break.

Now, I just wrote this response off the top of my head.  I'm sure readers can improve upon it - please post in the comments.  And I'm sure others have different approaches to dealing with disappointed parents, so please read comments to hear from them, too.  I'm not the only teacher with good answers for the Smiths.  Above is merely my personal approach to such a conflict.

The important components of my reply:

1. I've said nothing negative about Johnny - instead I've acknowledged and validated the Smiths' view of their son as diligent and positive.  Even if Johnny were evidently not a diligent student, I wouldn't point that out, I'd just find something else to compliment.

2. I've said nothing about the grade.  The point here is that no grade is a reflection on the character of a student; no grade, even if "bad" in the parents' minds, should be nitpicked.  My refusal to engage in discussion of grades means the parents have no room to advance a counterargument - the next email can only regurgitate the same points already made, which will look even more whine-y and arrogant the second time 'round.

3. I have responded directly to Mr. Smith's inquiry, with a polite, neutral, but unambiguous "The semester grades are final."  Mr. Smith can't complain that I ignored or talked around his request to reevaluate his son's grade.  I would not hedge here - sure, there are exceedingly rare circumstances under which grades may be revisited, like... like who cares.  None of those circumstances applies right now.  So I won't bring up their existence, or the Smiths will spend an enormous amount of energy convincing themselves and others that they do apply.

4. The final sentence is positive in tone, subtly reminding Mr. and Mrs. Smith that in fact I do care about Johnny, I root for his success, I'm proud of him so far and will continue to be... and that we still have another semester in which Johnny and I have to work together.  No need for an umpire to threaten the batter with "You argue with me again and the strike zone will reach to the moon."  Better to just remind him or her, "Here we go, batter, we've got seven innings to go.  Let's play ball."  The actual effect of each of these two sentences is about the same; so use the gentler words that will obtain the desired result.

What about the Smiths' next move?

You as a teacher worry perpetually that parents will complain to an administrator, which in the best case sucks your time away from teaching, and in the worst case leads to hostile conversations and actions involving Your Boss.  We all know that the Smiths will likely carry their fight for a revised grade past you to your administration.

By visibly, intentionally, and unambiguously copying your response (which, of course, includes the text of the original email) to those bosses, you've preempted the potential appeal.  You've put your administrators in the best possible position to support you.

See, even a good, supportive administrator has an obligation to reply to polite inquiries from parents.  If that administrator is unaware of your initial conversation before hearing Mr. Smith's further complaint, (s)he will reasonably come to you to hear your view.  While that's 100% responsible behavior on the administrator's part, it sets up an unfortunate false equivalence between your position and Mr. Smith's - the administrator seems to be adjudicating a dispute between two litigants, each of whom has equal standing.

By copying the administrator on your reply, you've indicated to everyone (including Mr. Smith) that Mr. Smith's concern has been resolved to your satisfaction.  When Mr. Smith tries to go over your head, the administrator would have to directly and publicly contradict your decision in order to take Mr. Smith's side.  Good administrators won't do that; even bad administrators don't like to be seen publicly undermining their teachers.

So if you don't trust the competence or goodwill of your bosses... still copy them on the response, but openly include a copy to an administrator you do trust.

How do you respond to the follow-up note from Mr. Smith, which is copied to all, making further arguments about how much you have hurt Johnny, pleading with even more emotionally charged language?

You don't.  See, you're in charge.  You don't need to win an argument with Mr. Smith.  You don't need to convince him that he shouldn't challenge your authority as teacher, or that Johnnie will do just fine at college whether he gets an A- or a B+ this semester.  Your job isn't to bring forth righteous justice, it's to bring forth peace.  Let Mr. Smith have the last word.  Let him look petty.  Carry on in the next semester as if nothing has happened.

## 17 December 2017

### Do I enjoy giving exams? Yes.

The question was once asked of me, in a most appropriate manner, whether teachers enjoy giving exams.  The asker seemed brain-dead from his first night of studying, knowing that he had a full week of hard academic work in front of him.

Though he was too polite to verbalize the expression in his face, I suspect his deeper thought was: “Why do teachers put us through this heck of cramming?  Do they get sadistic pleasure out of it?  Do they enjoy tormenting their students?  Weren’t they students once?  WHY DO THEY DO THIS TO US?”

Since the questioner was so polite, since he truly seemed curious about the answer, and since I’m sure much of the student body asks themselves the same question three times* each year, I think the question deserves an answer.  I can’t speak for teachers in general, nor even for teachers at my school.  But I’ll answer for myself:

* my school is on trimesters, not semesters

Yes, I enjoy giving exams, despite the enormous amount of work they create for me and for my students.  But probably not for the reasons you might think.  Put yourself in a teacher’s loafers for a moment…

When exam day arrives, I have dedicated the previous eleven weeks of my life to teaching physics.  I am “on duty” virtually nonstop when school is in session, especially in the fall when learning physics is most difficult for my classes.  When I’m not actually in class teaching, I’m grading assignments, preparing lectures and demonstrations, writing problems and assignments, helping students… You can ask my wife – all trimester I am thoroughly, monkishly, devoted to helping my students learn physics to the best of their ability.

The exam is an opportunity to find out how well I’ve done teaching, and how well my students have done learning.  I want to know – did those long hours, those occasional interminable days and early mornings, did they pay off?  Did I really succeed in develop every student's physics skills?  What can I do better?  What can THEY do better?  What did we do well?

I think of my teaching job much like a coach’s job.  Did Mr. Hale enjoy the state cross-country meet?  Well, of course he did.  Even though it was the runners who performed, not the coach, Mr. Hale still saw the fruits of his team’s months-long labor in the “final exam”  of the year’s last race.  As in every season, he rejoiced not only for the runners who placed near the top of the league, but also for those who showed dramatic improvement under his tutelage.

In my class, then, the trimester exam is equivalent to the biggest game of the season.  I am cheering for everyone to do well.  I know from experience that most students, in fact, will do well.  I'm ready to use the exam as a learning tool for those who don't do well - as a learning tool for that student, and as a learning tool for me as I figure out how to help that student do better next time.

Do I enjoy a student's poor performance?  No.  Yet I thoroughly enjoy the successes, which vastly outnumber the failures; and even when students don't do well, I enjoy the process of finding out how good I and my students have been.

Now let me throw this question back at the student who asked it.  Do you enjoy the state cross country meet?  Because if you don't work really, really hard during the season, you're not likely to win the race.  Even if you do practice well, a freak trip-and-fall could wreck the performance that you've worked all season to produce.  And you could lose not because you didn't work hard, but just because another runner has more natural, raw talent than you do.  YOU COULD FAIL.

So, grasshopper:

When you feel the same nervous, excited anticipation for your final exam as for the state championship...

When you develop the same discipline in academics and in sport to prepare every day throughout the season, not just the night before...

...then you, too, will enjoy exams.

## 15 December 2017

### A guide for useful and successful semester exams

A school community too often anticipates semester exams as they would an auto-da-fé.  We teachers know that the purpose of exams is not to torture, but to provide a touchstone in the learning process.  When the semester exam process is executed* well, students derive confidence from the skills they demonstrate, providing a foundation for an increased pace and difficulty in the second half of the year.

*hah!

It’s part of our job, though, to help the students develop an exam-positive view of the learning process.  Our attitude going in, as well as the specific things we ask our students to do in preparation for the exam, must be in line with the true purpose of exams.

Here, then, are four ideas for setting the right tone so that you get the most out of your semester exam.

(1) Insist on serious attention to every assignment throughout the year, so that the exam doesn’t seem like the teacher playing “gotcha”.

Your goal should be to have your students ready for the exam without any extra studying.  Hold every student accountable for not just doing each assignment to get it done, but for doing it right.  When students miss something important on a homework problem, give a similar question on an upcoming quiz.  Require the student who bombs this follow-up quiz to attend an extra-help session to do the problem right.  Make it more trouble to do assigned problems wrong than to do them right.

Why? Ask yourself, what does effective studying for an exam look like?  Very much like the process above.

(2) Be careful not to study-shame

How does this sound from the director of the school play with opening night imminent: “Make sure you practice your scenes 100 times each tonight; those of you who do might hit all your cues, but if you don’t spend that time tonight, your performance will suffer and you’ll trip and fall during the dance number in act 1.”

Even if you don’t intend to shame or threaten your students, that’s what they hear when a teacher explicitly invokes the consequences of unpreparedness.

Recognize that students - especially juniors and seniors - will not change their exam preparation habits based on entreaties from you.  You think that the student who hasn’t done serious preparation for an exam in any of five classes and two semesters per year for four years will suddenly say “Oh, Mrs. Lipshutz says it’s extra-important to study for physics, I’d better do that.”?!?  If our mere words have that much influence over recalcitrant teenagers, we should be in politics or marketing, not teaching.

More likely, we lose political capital and put emotional distance between us and our students.  Teenagers don’t like to be told what to do, even if what they’re being told is undoubtedly in their self-interest.  They tend to rebel.  There’s no point in complaining about this aspect of teen psychology; we might as well kvetch about how the Bengals always punt on 4th and short.  We’re right, but powerless in the face of human irrationality.

(3) Use positive rather than negative incentive to promote exam preparation.

Fear of failure is a serious obstacle to success, whether on this particular exam or in physics generally.

If you give students no direction for exam prep, they will default to study modes that have been successful for them in the past* in math and history courses -- memorize as many facts from the textbook as possible, practice and memorize algorithms that they’ve used in class before.  Such approaches to preparing to do physics are not even wrong - they are actively detrimental to a student’s progress, and to the progress of the class as a whole.  Diligent students who already struggle with difficult concepts undertake their not-even-wrong study regime, bomb the exam, then have ammunition when they tell parents and administrators how your course is too hard and unfair - they studied so much, and still didn’t do well.

* Or at least, modes that parents and teachers believe on faith to be successful

I’ve found success giving students something straightforward and useful to do for exam preparation.

“Here is a review exercise consisting of twenty multiple choice questions.  Please fill out the answer sheet on your own, without assistance from anyone - no questions at all, just like on the exam.”*

*(If you can’t trust students to answer these questions on their own, give time in class to do them.  It will be well worth the effort.)

“Once you’ve answered all the questions, I’ll scan your form.  Extra credit will be awarded to anyone who writes out a thorough justification for all of the problems that they missed.

"Collaboration with me and classmates on these justifications is encouraged.

“I’m offering a study session after school on Wednesday.  Pizza and nachos will be available.  Come on by – it’ll be fun, and I’ll help you with any questions that are still confusing you.”

Look what’s accomplished by this approach.

Instead of requiring that students take a practice exam, instead of lecturing about how good an idea it’d be to take a practice exam, you’re couching the practice exam as an optional “extra credit” exercise.   Which approach do you think gets more willing cooperation?  Yeah, I don’t believe in extra credit, either, but I *do* believe in students undertaking exam preparation enthusiastically rather than reluctantly.  (And students who didn’t complete the practice have no standing for a post-exam complaint to parents and administrators - wait, you “studied for hours” but you didn’t do a small practice assignment for extra credit?  Not likely.)

You focus the students on what they really need to study, because they spend most of their time correcting the questions they missed.  Without such guidance, students may spend a couple of hours doing problem types they already are good at, while ignoring the topics or styles of problem on which they struggled.

Some students will work themselves into a panic, thinking there’s always more that can and should be done to prepare for an exam.  These students are put at their ease: “Once you finish this exercise, you are ready.  There’s nothing more you should or can do to prepare.”

Some students will tend to do nothing at all in preparation for the exam, even though a review of some sort would be useful for them.  These folks will tend to do an extra credit exercise that all their classmates are doing, too - just to be social, if nothing else. And lo, your recalcitrant students have studied, without even a nanopascal of pressure from the teacher.

(4) Make the exam itself – and the follow-up to the exam – worthy of your class goals.

But that’s an article for a different time…

## 06 December 2017

### Don't play cops and robbers with phones.

In a September post, I explained briefly how I deal with students who ask to go to the bathroom.  It's very simple: I say, please don't ask, just place your phone on your desk and go.

The comment section of that post became active, and brought up an important piece of teaching philosophy.  First, here's my follow-up comment explaining the purpose behind the "just put your phone down and go" approach:

Note that I'm not in any way making the rule "no texting in the bathroom!" Uh-uh. That sounds condescending, it gives students ideas, and it worries the rule-followers.
No rules here, in fact I'm giving students freedom from rules - in other classes, they feel oppressed that they have to ask permission to exercise a simple bodily function, and furthermore that the teacher is likely to nag them about their body's timing. Here, they are free to do as they need to.
Yet, trust but verify. Since the phone goes on the desk as a matter routine (not rule), there won't be any texting in the bathroom. Then it's my job to hold activities interesting enough to minimize using the bathroom as an excuse to relieve the boredom of class.

Later on, Dean Baird brought up how students will, inevitably and frustratingly, escalate a battle with their teacher passive-aggressively:

Seems reasonable. Of course, students intent on "phoning out" while using the hall pass will equip themselves with "burners" to satisfy instructors who adopt such strategies. Hall pass use is a sticky wicket; a puzzle not so easily solved. In courses populated y highly academically challenged students, some find a daily need for hall pass usage. And any kind of restriction is virtually impossible to implement. Offering carrots for non-usage works only with students concerned with academic performance. "Pretty good" and "Good enough" solutions are the best we can realistically hope for.

Dean's right that some students will see bathroom texting as a game, to see how they can beat the system and "stick it to The Man." (True even when The Man is, in fact, The Woman.)

And my response - LET THEM. As soon as we engage as cop, the students engage as robber.

I say "please leave your phone" for the same reason the audience is asked "please turn off your phones" before a stage play begins. It's all too easy for anyone, adult or teenager, to fall to the temptation to real-quick check that important text, or to answer a buzzing phone from a number we recognize. Leaving the phone in the classroom, turning off the phone before the performance eliminates that temptation and helps the class/audience maintain an extended period of focus.

So what do we say about the audience member who smirks and pulls out a second, burner phone on which to text during the play, and then tells the usher "hey, but I did what you said, I turned off my phone, you can't kick me out, I'm gonna sue?" That's a problem that goes beyond techniques to manage people; such behavior is no different from extending a middle finger to everyone, including the performers and the other audience members. This dumbarse needs to be ushered away toot sweet without discussion.

But the existence of the willful fool doesn't mean that we should change our respectful approach to the rest of the audience. "Okay, folks, last night we had a guy texting in the middle of the performance and thumbing his nose at the house rules. I'm sick of you audience people not being able to keep away from your phones. So tonight, we're going to collect phones before the show, and anyone who sneaks out a second phone will face criminal charges. Here's Chief Wiggum at the front ready to enforce those rules. We're not playing around, got it?"

And that's what teachers sound like when they make draconian rules to deal with one or two uncooperative students. The guy using the burner phone in the bathroom thinks he scored a point against you. But we're not keeping score. Find a way to deal with the individual that doesn't involve class rules. Or just ignore him - the rest of the class may laugh with him, but if no one else is using burner phones, maybe it's not that important for Batman to defeat the Bathroom Texter. :-)

## 04 December 2017

### Motion graphs - pay attention to subjects and verbs

The biggest mental block toward understanding motion graphs is the idea of a representation: that features of a graph indicate real motion of a cart.  It takes careful teaching on our part, and mental discipline on our students' parts, to connect the vertical axis value of a velocity-time graph (or the steepness of a position-time graph) to how a cart actually moves.

The best tool I've discovered to help students make these connections is the written word.  I hand out the facts about motion graphs, and we do my version of a graph-matching exercise.  But students can't just get the answer right and call it a day... they must write their justifications using (a) a fact from the sheet written out word-for-word, and (b) how that fact connects to the graph their working on.

And in this way I can nip faulty reasoning in the bud.  I make them rewrite immediately when they tell me "the cart slows down because the graph says," because there's no fact of physics involved there - even if they're right that the cart slows down.

More importantly, I pay careful attention to subjects and verbs.  The graph can change steepness; the graph's vertical axis value can change.  The steepness and vertical axis values represent how a cart in the classroom moves.  It's important that no one says "the graph moves" or "the cart's steepness changes."  When I see those statements, I ask the student to rewrite with the correct subject and verb.

Students at first find this nitpicky.  So what.  By now they should (and do) know that physics isn't about right answers, physics is about communicating an understanding of how the world works.  After a few classes, the class is really quite good at interpreting motion graphs, and they stop confusing the features of the representation with the real, live motion of a cart.

The following is a note I sent to my 9th grade class last night as a reminder of the care they must use in their written responses to motion graph questions.

Please consider carefully the subjects and verbs you use on your motion graph justifications.

"The cart moves closer to zero on the vertical axis, and so slows down" makes no sense.  As you've seen, the cart moves on a track in the classroom; the cart cannot move anywhere "on the vertical axis."

"The graph moves upward on the vertical axis" similarly makes no sense - the graph does not move, the graph is still on the paper on your desk.*

* Unless you threw it upward or something.

The cart moves; the graph does not.  The vertical axis of the graph indicates how fast and which way the cart moves.

You want to say, "The vertical axis values get closer to zero, so the cart's speed gets closer to zero."

(And you never, ever want to use the word "it."  Write "I didn't say it" on top of your problem set for an extra credit point.  Don't tell others this, keep it to yourself!  :-)  )

## 24 November 2017

### Multiple options for a Physics C multiple choice question about capacitors

A parallel-plate capacitor is filled with air. Each plate has area 10 cm2. The separation between plates is 2.0 cm. The top place stores +200 nC of charge, while the bottom plate stores -200 nC of charge.

Above is just the stem of the question I'm writing for the upcoming edition of 5 Steps to a 5: AP Physics C.  Consider it a "goal-less" problem for a moment.  What could be figured out?

The first and most obvious answer is to calculate the capacitance of the capacitor using C = εA/d.  And yes, that is a calculation fraught with pitfalls, as the plate area and separation have to be converted to SI units.*  But carrying out this sort of calculation is not the point of introductory physics.  If you really want a correct calculation, don't ask a first-year high school student with a calculator - use Wolfram Alpha.  More importantly, I don't expect even strong students to be able to calculate the right answer in about a minute.

* Or I suppose you could convert epsilon-naught to F/cm.  Whatever floats your boat.

Better, more authentic questions might involve semi-quantitative reasoning.  What happens to the capacitance when I double the plate area? Plate separation?  Charge storage?  Using C = εA/d, you can see quickly that the capacitance doubles, halves, or remains the same because the relevant equation is in the numerator, denominator, and not involved, respectively.

A further step could involve graphs - what would an experimental graph of capacitance vs. plate area, plate separation, or charge storage look like?  Linear, inverse, and horizontal, respectively.

Or, we could go one step beyond single-equation reasoning to ask how the electric potential across the capacitor changes depending on plate area, plate separation, or charge storage.  This question involves combining two fundamental relationships: C = εA/d for the capacitance, and Q = CV for the voltage.  In this case doubling plate area doubles capacitance, but halves voltage; doubling plate separation halves capacitance and doubles voltage; doubling charge storage doesn't change capacitance but doubles voltage.

(And you could even ask about the electric field inside the capacitor, which is uniform and equal to V/d.  Or the field or potential at several positions inside the capacitor.  Or...)

A fun exercise might be to present your class with the stem above and some whiteboards with, say, 10 minutes remaining in class.  Then tomorrow's daily quiz would include two multiple choice questions based on this stem, with only two minutes to answer.  I'll bet you get (a) good conversation amongst the students, (b) mostly right answers on the quiz, and (c) everyone finishing the quiz with lots of time to spare.

## 16 November 2017

### Pivot Interactives - these videos are so worth the money.

 Screenshot from Pivot Interactives
As you might have noticed, my all-time favorite internet physics resource Direct Measurement Videos has migrated.

Vernier now is selling access to "Pivot Interactives", for \$150 per year or \$5 per student, whichever is greater.  On one hand, I wish the National Science Foundation had stepped up with a seven-figure grant for primary DMV creator Peter Bohacek so that he could continue to provide these resources free to physics teachers.  I mean, I can list probably 102 NSF grants that flush my tax money down the toilet. Nevertheless, I'm happy that Vernier has seen the extraordinary value in these exercises to keep them alive.

The question for the physics teaching community is, then, do we spend the couple hundred beans to get access to Vernier's new site?  For me, the answer is definitely "yes."

I've made it my official teaching goal this year to replace as many textbook-style homework problems as possible with Pivot exercises.  Since Peter and company have been hard at work adding to their video library, I'm finding this goal easy going.

Take, for example, the car-around-a-traffic-circle problem.  I've always started my circular motion unit there, as it's a situation all my students have experienced.  I ask, how fast can the car go around the curve?  We discover that the maximum speed depends only on the curve's radius, g, and the coefficient of (static) friction.

Great.  But this is an abstract "imagine if" problem.  I can't take my students to a traffic circle for experimentation - the nearest one is 20 miles away, and is too busy for fooling around, anyway.  All I can do is suggest that the yellow suggested maximum speed signs don't include a mass variable - they say "max 25 mph", not "max speed in mph is 0.025 times the mass of your car in kg."  Interesting... but not experimental.

Well, look at the screenshot at the top of the post.  Peter took his drone to a traffic circle.  He drove the gray car around the circle at a speed that was always on the verge of slipping.  When he imported the video, he included tools to find angles around the circle, the radius of the circle, and a frame-by-frame timer.  I can't do this experiment, but Peter can. And did.

So for a homework problem later in the circular motion unit, I link the class to this Pivot Interactives video.  The site allows me to customize the assignment - the default is quite good, but I can add or eliminate questions and guidance.  For me, I like a clean prompt like "Determine the (maximum) coefficient of static friction between the car's tires and the ground."  The site allows students to input their solution and reasoning directly in the space provided; you can then scroll simply from one response to the net, awarding points if you'd like.  I prefer to have students answer on paper, but that feature seems to work as well (paper not provided by Vernier).

There's so much more that Pivot does.  I prefer the simple open-ended "determine this parameter" exercises.  But Pivot also has some modeling exercises, providing an easy graphing interface that allows students to make and linearize plots.  These multi-layered videos allow you to change multiple parameters.  The prompts guide students through what essentially is a complete lab exercise which you might never be able to do in your own classroom; or, a lab exercise you don't have the time to do in your classroom but can assign for homework.  I know that Vernier has gotten some serious physics teaching experts, including Kelly O'Shea, writing these exercises.

And Peter is adding videos every time I look.

Look, I know it's disappointing to have to pay for what had been available for free.  And I generally don't recommend paying for physics content on the internet.  Nevertheless.

Vernier has hired the varsity for this project.  Everything I see on the site is something that makes me say "wow."  Pivot cannot replace a physics teacher doing active lab work in the classroom, because nothing can.  Using Pivot gets students as close as I believe it is possible to come to an online laboratory experience.  I highly recommend.

GCJ

(Note that Peter and Vernier have not paid me in any way for this endorsement.)

## 15 November 2017

### Umpires, and Training Students to Grade Each Others' Daily Quizzes

The most effective tool I've ever discovered for 9th grade conceptual physics is not the daily quiz - it's the grading of the daily quiz.  Students trade papers, pick up the red pens I provide, and mark right or wrong as I go over each answer.

The purpose here is pedagogical, not logistical.  I'm perfectly capable of rephrasing these questions as multiple choice for ease of grading; and it's not like I don't have time to grade these simple quizzes, anyway.  No, the reason we trade and grade daily quizzes is that students pay attention to each question threefold:  once when they take the quiz, once to figure out how to grade the quiz in front of them, and once to think about whether they themselves got each question right.

It's essential, though, to establish an appropriate tone and ground rules in order for trade-and-grade to be successful.  Here are some principles that my class learns that make the daily quizzes work.

(1) We are a team.  Daily quiz performance is just like a football team's conditioning sprints.  Teams have fast and slow players.  Good teammates respect the effort and talent of those who are the fastest; good teammates encourage and respect those who are slower, too.

The team atmosphere must be established from the first day of school.  The highest form of sin in my class is to denigrate  a classmate for a wrong answer.  Even negative body language directed at another student - a teammate - is unacceptable.

Because we are a team, we can depersonalize the grading process.  A grader is acting as an umpire for the team.  Umpires don't call people out because they dislike the players, they call safe or out honestly based on their judgment, and on a shared respect for the game.  A good team doesn't want a biased umpire, even if the bias is in their favor - they want to win or lose with integrity.  Those who play sports know that a tainted win is nothing to be proud of.

(2) Don't look at or talk to the person grading your paper.  You have enough to focus on with the paper you are grading.  Trust your classmate to evaluate your work appropriately.  No one respects the player who continually tells the umpires how to do their job.

(3) Don't talk to the person whose paper you're grading.  Imagine an umpire who, presented with a close pitch, asks the catcher: "Hey, did you mean for that pitch to be over the outside corner?"  The batter would go berserk!  So students should never ask "hey, did you mean this line to be straight?"  If you can't tell, mark it wrong.  The burden is on the student writing the quiz to communicate clearly.

(In my AP classes, I ask regularly: "Are you allowed to travel with me to Kansas City in order to accompany your exam from reader to reader explaining what you meant?  No?  So don't get in that habit now.")

(4) Ask for help on borderline calls.  The class is instructed to raise their hands and read me any answers they're not sure about.  Not generalities like "What if the student kinda hinted at the answer?"  I ask the student to read, verbatim, what is on the page.  I then make a call one way or the other, and we move on - just like an umpire.

And if the student who wrote the answer tries to chime in what he meant, I politely say, "Since the answer needed clarification, it must be incomplete.  Please mark it wrong."

(5) Be transparent.  Just as a league doesn't assign the same umpire to the same team too many times, be sure you're mixing up occasionally who gets whose paper.  Have the graders write "graded by" and their name at the top of the page, and make it clear that accurate grading is a skill that you are assessing.  Ask students to place their own pencils and pens on the floor while grading goes on, so that the only writing utensil available is the red pen - that way, it's not possible to change an answer dishonestly.

If you suspect someone is grading inappropriately to benefit (or hurt) a classmate, don't ignore it, but don't get preachy!  I've had small issues early on - students giving credit when they shouldn't, hoping to curry favor with a classmate.  Those were easily and definitively solved when I asked publicly but earnestly, "Luke, can you please show me why you marked this answer correct?  I think it's clearly wrong, but am I missing something?"  That's all it takes.

(6) Make instant replay available.  We have a standing rule - students may never, ever, argue or discuss with the person who graded their paper.  But they may circle in red a question they think was graded inappropriately, signaling me to take a look.  They can't talk to me, they can't tell me what they meant, they can just circle the question.  And, if the grading was indeed incorrect, I will change the score and let both student and grader know what happened and why.

Early on, usually I get a bunch of students circling their answers, hoping to argue for a better score.  That ends quickly when they can't talk to me to lawyer up.  If someone can't stop arguing and circling obviously wrong answers, I tell that student that I'll take off an additional point for each answer they challenge incorrectly.  (Kind of like how in the NFL a coach loses a time out if he incorrectly challenges a ref's call.)

Thing is, students are more careful graders than I am.  The process is quite fair - even moreso than if I combed through all the quizzes.

The most important point to keep in mind, though, is that the grading process is not even about fairness.  The goals are twofold - to provide an arena in which students are paying attention to physics facts, but also to establish that physics has right or wrong answers.

Within a week my class has stopped arguing with me about grades.  They know what my answer will be - it is the student's responsibility to communicate correct physics the first time, in writing.  They've seen that the entire class is held to the same standard, that no one can sycophantically beg for points after the fact.