Physics Walks - weekend of Nov 27-28-29, 2020

Do you have a question, story, or comment about physics teaching?  Let's take a physics walk.

Hi, all.  Students left last Friday, grades are done, and we are not traveling for Thanksgiving.  I'm having a hard time convincing myself to stop eating cheese on the couch.  

At the AP Physics reading, there's a group who spends their hard-earned lunchbreak walking the streets of Kansas  City.  I miss those physics walks.  We usually talk about physics, physics teaching, fun and funny stories about our schools and students... 

This weekend, I'm setting aside several hours when I'd like to be walking around campus with my dogs.  This is a great time to talk to me about physics teaching.  No obligations, no strings attached - sure, your questions might inspire a future blog post or podcast, but truly, I'm just looking for a friendly reason to get outside while I'm quarantining.  All physics teachers welcome, whether you've been in one of my institutes, or whether you've never met me before!

To sign up:

1. Go to this google doc and find a time that works for you.

2. Follow the instructions to send me an email with contact info

3. I'll call you this weekend while I'm walking!

Field Technician Mark May - when a job becomes a Calling

To me, helping students understand how the natural world works is sacred.  Broadcasting soccer or baseball or football is sacred.  There is a Right Way to do these things, and I live in pursuit of discovering, executing, and sharing that Right Way.  

The people I like the most, the people I understand the most, the people whom I most enjoy as colleagues are those who have a laser-focus on an outcome that is sacred to them.  I want to be a part of a community of individuals with diverse interests, who take those interests not just seriously but religiously.  For many boarding school employees, school isn’t just a job, it is a calling.  

This isn’t just about teaching.  My wife Shari’s sacred goal is to create beautiful and functional pottery in a collaborative studio atmosphere.  I know nothing about pottery, I don’t even like looking at it  - but I enjoy hearing her talk about her process, about the Right Way to organize a studio culture.  In college, my roommates were into X-men, home improvement shows, and Bob Ross.  I have no personal interest in any of these things.  Yet I loved hearing them talk about the backstories, the little nerdly details they obsessed over but few other people would notice or care about.  

Within our community, I’ve always held deep respect for those uber-professionals beyond the faculty who go way beyond the minimum job requirements.  Cronin Warmack our faculty technology pro, Richard Johnson our long-serving head of housekeeping, Gary Brookman and Don Carlson the golf course groundspeople, and more.  They don’t just do the job ‘cause they’re paid, they do the job because it has to be done; and it has to be done Right.

In 2002 I was assigned to coach JV baseball.  We practiced on a bumpy, out-of-the-way field next to a cow pasture.  It was well kept, way better than the pitted hayfield monstrosities I played on in northern Kentucky in the 1980s.  Yet I had low expectations.  Who gives a rip about JV baseball, beyond the kids on the team and their parents?  The varsity coach sure didn’t.  

On our first game day, I found the field transformed.  Okay, we weren’t at Dodger Stadium - you can’t hear the lowing of cattle in the background there - but the behind the scenes groundskeeping effort had been equivalent to that of a major league crew.  The lines were constant-speed-position-time-graph straight, fresh, and immaculate, including the chalked parts on the dirt as well as the painted parts on the grass.  Even the obscure details like the coaching boxes and the batters’ boxes were just right.  The pitcher’s mound could have been used as a model for the rulebook’s “1 inch drop for every 1 foot towards home plate” language.

I asked the athletic director: who had taken such loving care of our field?  “Mark May,” he said.  I found this guy Mark and told him how much I appreciated his detailed work.  Mark seemed a bit surprised that I had noticed some of those details, but he was happy that I had.  

Mark prepared our field exactly like this for every JV home game.  Of course, he prepared the varsity baseball field identically.  And the lacrosse fields.  And the other parts of the fields like the benches and the bleachers and the screens and the fences and the nets and the equipment cupboards and… everything.  

At other schools, the varsity programs, or the programs coached by powerful administrators, get the high-class treatment while the teams lower on the sports hierarchy get the shaft. Me, I prefer to be low on the sports hierarchy.  I love coaching JV, I love running an intramural program for those who can’t play interscholastic sports, I love broadcasting lower-level games.  Why?  It’s the purest sport there is.  My school’s most iconic mural reads “Effort in sport is a matter of character, not reward.  It is an end in itself, not a means to an end.”*  Too many varsity players are focused on sport as a means to a college scholarship, personal glory, or social status.  JV players are almost all there for the love of the game.  Yet, lower-level programs aren’t always well treated.  ADs and staff and even coaches can generally keep their jobs while ignoring the sub-varsity teams, as long as they kiss important people’s arses and take care of the high-profile teams.

*I would and do say the same thing about effort in physics.

In the late 2000s, an AD put pressure on the staff and on Mark to focus on the favored sports.  Nevertheless Mark always, always made sure my insignificant programs had what we needed.  It would have offended his very soul if a field weren’t exactly right.  Every team mattered, every player, every coach, every visitor.  

When I first ran an intramural program in 2002, I described to Mark the non-standard field markings we needed for flag football and for soccer.  He made notes, lined the fields perfectly… and kept those notes. 

He kept those notes for 18 years.  How do I know?  We didn’t run an intramural program after 2008.  But this year, by necessity, the freshmen and sophomores needed an intramural league because interscholastic competition was impossible in the age of COVID.  I volunteered to run this league.  I told our (wonderful current) AD that Mark would know how to set up our fields.  Sure enough… Mark pulled out his notes, and the fields were exactly as requested.  As requested 18 years previously.

Then there was the Big Football Game one year.  We had probably 7000 people on campus for the yearly rivalry game.  The bleachers were packed, as were the auxiliary bleachers, the standing room in the corners, the area behind the end zones… fans were everywhere.  The game ended, the team and the parents got together on the 50 yard line, everyone savored the beautiful fall weather, the camaraderie, and the afterglow of victory.

Except Mark May.  In the bleachers he held a giant trash bag with one hand while his other picked up soda bottles and chip wrappers, one at a time.  Why was he there?  The cleanup job wasn’t urgent - night was falling, the campus was about to empty for a couple of days.  Had Mark waited until Monday, the entire gounds or housekeeping staff could have been mustered for this herculean job.  But Mark didn’t want to wait.  

I asked Mark for a trash bag, and I helped out for half an hour or so while the sun dropped behind the bleachers and below the horizon.  We had marched our human vacuum through a significant but small fraction of the main bleachers.  Mark had emptied Grand Traverse Bay, but Lake Michigan itself was still full.  I grabbed a couple of full bags, said goodbye to Mark, and headed to the dumpster - and home to my wife, who was probably wondering whether the silly sportsball game had gone into double-time or something.  Mark, of course, kept going, even as dusk turned to astronomical twilight.  He had a job, it had to be done, it had to be done Right.

Mark May died Friday.  He was found in his office on campus. 

Mark had been Woodberry’s Field Technician for 35 years.  He worked to the job, not the clock - which meant I saw him lining fields, carrying equipment on numerous early mornings, late evenings.  Sometimes I marveled to myself, “wow, he’s always here!”  

Mark will always be here.  Man’s not dead while his name’s still spoken.  

 

Come-and-show-me activities online - try classkick.com

In class or consultation, I regularly have students line up to show me their work. Then feedback is personal and meaningful right now; and, advice to one student is overheard by those in line, who quickly adjust their work based on that advice.

I hated that I couldn't use this come-and-show-me style online in the spring.  I tried several ways to simulate it, and none was satisfactory.  But...

Physics teacher Tiffany Fuhrmann suggested I try classkick.  It's free - you can use your google account to sign in.  You create an "assignment" simply by uploading a pdf.  Students don't have to register - they just input the class code that classkick creates for you.  (You *can* upload a list of your students, but you don't have to - anyone who joins with the class code is in.)

Then, with minimal effort, you click "view work" to get a screen like you see below.  Each student's work is in a separate row.  Each column is one page of the PDF you uploaded.  Students can respond by typing, writing (on an ipad, phone, touchscreen), uploading pictures, or perhaps other ways I haven't discovered.  

I can see students working... I can click on one box and up comes the student's work.  Then *I* can write or type feedback!  There's so much more here that I haven't quite discovered, including student-student collaboration... if we're all on zoom, everyone can hear my feedback, and I can have an open breakout room for students to talk amongst themselves.  

This is revolutionary enough to share with others.  I tried it tonight with some physics teachers - it was way simpler than I anticipated, and devoid of the technical glitches that I fully expected.  

What does "r" mean in equations for gravitational force and centripetal force?

On one hand, that's easy for me to answer: 

In the gravitational force equation F = GMm/r,² the r represents the distance between the centers of the two objects.  I often use the variable d to emphasize this meaning.

In the centripetal force equation F = mv²/r, the r represents the radius of the circular motion.

These facts are also easy for students to read and recall.  It's not as easy for students to put into practice.  They see the letter r, hear "radius," and plug in any random distance they can pluck out of the problem stem.

And, this <pop> pull-a-radius-value-out-of-their-tuckus method is very often successful in a gravitation problem.  When an object is on the surface of a planet, the r value is in fact the planet's radius.  When a satellite undergoes a circular orbit around a central planet, the orbital radius r is in fact the same as the r distance between the satellite and the planet's center.

So why does it matter if students truly understand the difference between these two meanings of r?  In what possible physical situation in introductory physics does this difference even matter?  Here's one.

Two stars, each of equal mass M, maintain a constant distance x apart and rotate about a point midway between them at a rate of one revolution in every time t₁.

(a) Why don’t the two stars crash into one another due to the gravitational force between them?

(b) Derive an expression for the mass of one star.  Use given variables and fundamental constants only.  You must annotate your calculation – if your response has no words, you will redo it from scratch in consultation.

This is a difficult question for students to conceptualize, especially because while we've done plenty of straightforward orbit problems, students have very often remembered comforting algorithms and not necessarily internalized physical meaning.  And I won't answer questions from students before they turn it in.  (They can discuss and argue with each other as much as they want!)  In class the day this problem is due, I don't start by "going over" how to solve it.  And on that model, I won't simply go into my solution here.  Instead, I'll show you the quiz with which I begin class:

For #1, r in this equation represents the distance between the centers of the two planets.  The problem says explicitly - the stars are always 8.0 x 10¹⁰ m apart.  (Most common misconception:  because in every problem they've previously done a satellite orbits around a central planet, they think that the "point midway between" the stars is the location of a planet of some sort - or that this "midpoint" is what exerts the force on the star.)

For #2, r in this equation represents the radius of the orbit.  The problem says explicitly that the stars "rotate about a point midway between them".  Since the stars are 8.0 x 10¹⁰ m apart, the point midway between them is half that distance from one of the stars, or 4.0 x 10¹⁰ m.  (It's not correct that one star is fixed with the other orbiting around the fixed star.  That's not what the problem says, nor is it how binary stars behave.  And half of 8.0 x 10¹⁰ m doesn't mean divide the exponent by two: half of 80 billion meters is certainly not 4 hundred thousand meters!)

For #3, students know to use this equation when they know the period of an object's circular motion.  It comes from the fact that the orbital speed is constant - for constant speed, speed is distance/time.  The relevant time here is the period, the time for one orbit.  The corresponding distance, then, is the circumference of the circular orbit.  From geometry class, that circumference is 2πr, where r is the radius of the circle.  We want the same distance as in number 2, the 4.0 x 10¹⁰ m radius of the orbit.

For #4, the distractors are practically word-for-word from past student responses.  As I discuss the quiz, I pick an incorrect answer to explain why it's incorrect:

For (A), first we discuss and agree that the gravitational forces on each star are indeed a Newton's Third Law force pair.  Then I go to a student's desk, hand the student a string, and pull.*  Is the force of me on Mr. Chamberlain equal to the force of Mr. Chamberlain on me?  So is that a N3L force pair?  Yes.  Are we orbiting around each other in a circle?  No.  So the logic of choice (A) is not logic at all.

*In the Before Times, I'd clasp hands with a student and pull lightly.

Next I ask students to close their eyes.  All who chose this incorrect answer, raise your hands high.  Now put your hands back down, and open your eyes.  The point is for students to acknowledge their misconceptions. It's okay that they made the mistake - after all, in my class everyone gets an A- until the AP exam, quizzes are given and graded but don't "count".  I don't want students to feel shame for being wrong.  But more importantly, I don't want them to sour-grapes style convince themselves that they knew that and that they didn't really make a mistake.  No.  Own the misconceptions, then don't make them any more!

For (B), first we discuss and agree that the gravitational forces on each star are indeed a Newton's Third Law force pair.  I again go to a student's desk, hand the student a string, and pull. The forces on each of us are equal. Are we orbiting each other?  No?  Then (B) is wrong.

For (D), I ask a student to point to the midpoint between us.  Only objects can exert forces... so what object at the "midpoint" can exert a force?  No object.  So (D) is wrong.

And finally, for the correct choice (C), I draw a picture of the two stars orbiting.  I draw the direction of one star's instantaneous velocity, which is tangent to the orbit.  I ask about the direction of the net force on that star, which is toward the center.  Everyone sees that the velocity is indeed perpendicular to the net force.  And we discuss how that's a restatement of one of our circular motion facts: when an object moves at constant speed in a circle, its acceleration is toward the circle's center.  The velocity will always be tangent to the circle, which by geometry is perpendicular to the direction toward the center.