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29 October 2020

Physics Night In America: Sunday, November 8, 2020

Advertisement disclosure: This post is an ad for an event I'm hosting.

Are you a physics student who wants to ask questions outside of your classroom in a supportive environment?  Would you like physics help from the guy who wrote the 5 Steps to a 5 books?  (Do you know a physics student like that?)

Join me for the inaugural Physics Night In America on Sunday, November 8, 2020, from 7:00 to 9:30 pm eastern time.  I will broadcast via zoom, answering student questions, discussing your physics problems, setting up demonstrations, interacting with the audience... 

Think of Physics Night In America as a live improv show, but about your physics questions that you care about.  

How it works: 

* Register ahead of time, using the instructions below.  Registration for students is $30. (Teacher registration is free.) Attendance is limited to the first 30 students who sign up!

* After you're received your confirmation email from me, reply to that email if you'd like to discuss a physics concept, a specific problem, or a general topic.  All levels of high school and first-year college physics are fair game!  

* For everyone in attendance, I'll guarantee to somehow address the topics and concepts you request.    I won't do your homework for you, of course - I will help you understand your homework.  

* On November 8, I will send out a zoom link and password.  I'll be on a bit early to say hello, and to introduce you to my pet hippopotamus and her friends.  

How do students register?
(1) Click the "donate" button below (or in the left column of the blog).  It will take you to paypal.
(2) Enter $30.00 as the donation amount, either through paypal or credit card.
(3) Click "Add special instructions to seller" or "Add a note".
(4) In the note, please include your name, and the email address where I should send zoom information. 
(5) Fill in payment info and click "donate now".


Cancelation issuesIf you register then can't attend, contact me via email.  As long as I can replace your spot, I'll send a full refund; if I can't replace your spot, I'll refund all but $10.

How do teachers register?

Send me an email - greg underscore jacobs at woodberry dot org - with your name and your institution.  I'll put you on my list, and I'll send a link on Nov. 8!

22 October 2020

Mail Time: Linearizing a parabolic distance vs. time graph

An APSI participant writes:

I had a question regarding linearization.  We just did the free fall lab and a student graphed their data to get the parabolic curve of d vs t.  When asked what they would do to linearize, they stated to convert d to v and graph v vs. t.  

When they determined v, they did d/t for each point.  Then plotted that value for Vavg vs time.  Is this an acceptable way of linearizing?  

My first instinct is no, because the slope of the v-t graph, while still the acceleration, does not produce the value as it would if you did d vs t^2.  

The slope of their v-t graph was 5.15 m/s/s.  If they would have plotted d vs t^2, they would have a slope of 5.45 m/s/s with an acceleration of 10.9 m/s/s for g.

This is a common approach by early-in-the-year AP Physics students.  They did not do this right... 

By dividing just d/t for each point, they took the average velocity from the beginning - that's not how a velocity-time graph is made, and that quantity is rather meaningless in the experiment you describe.  Basically never divide the values of two data points!  :-)

What WOULD be a reasonable alternative approach to this experiment would be to make a legit velocity-time graph: by taking the slope of a tangent line to each point on the d vs. t curve.  Then plot those *instantaneous* speeds as a function of time.  That's a true velocity-time graph, for which the slope is acceleration.

Of course, that sounds way, way more complicated than just using lab linearization approaches of writing the relevant equation, then plotting data as it appears in the equation, d and t^2.  

Hope that helps!

20 October 2020

Notation for Newton's Second Law: F, Fnet, Sigma-F. And Julius Sumner Miller.

Alex writes in:

I was listening to Julius Sumner Miller lecture on YouTube while doing some work last night and noticed that he said F=ma. 

So my question is using the term net force something that teachers just use? Was it something that wasn’t used then but is common vocabulary now?

Glad you reminded me about Julius Sumner Miller (which is still what my brain wants to call my colleague Julius Reynolds, because until him I'd never actually known a Julius other than Orange Julius).  I should watch some of those videos with my AP class when we're online in December!  Make sure you watch the one where he puts random stuff in liquid nitrogen, and forgets some things in the carafe.  JSM is a big, big influence on my teaching style, and I'll bet you can tell.  He didn't have a pet hippopotamus named Edna, though, more's the pity.

That's an interesting history question, Alex... there are people who research the history of physics education.  I wonder... I copied Gardner Friedlander, one of my go-to folks for AP Physics history, in case he has something to add.  (He did - see below!)

Before we start - I am not discussing whether your personal version of Newton's Second Law is the best one.  Of course it is, and all others are inferior.  I will not publish any comments that talk about why one notation is best.  I'm discussing the various notations, their history, their pros and cons.  If you use sigma-F but I don't, that doesn't mean you're better than me or vice-versa.  Really. We all have our reasons for our notation.

I do seem to remember that when I grew up - and I watched JSM in high school physics class all the time, on VHS - the phrase in common use was just "F=ma."  I looked back at the earliest AP solution set I can find, and it says sigma-F = ma.  (There was no equation sheet back then, before the days of graphing calculators.)  The 1983 New York Regents exam says "a=F/m."  I'd love some evidence that my recollection of "F=ma" being the common statement is correct... certainly I think that was the non-physics-class zeitgeist meme that people would say.

When I started teaching in 1996, I always used the notation "Fnet," emphasizing that only the NET force could be set equal to ma.  See, I discovered quickly the common issue that students would pluck a force value from the problem statement, pluck a mass, and smash them together into F=ma to get an acceleration.  I didn't like the sigma-F notation because students tended to add force numbers without reference to direction when they saw that.*

*or they would cluelessly wonder "what's that big ol' squiggly E, I dunno, it's got an F next to it, I guess."

In the AP physics 1 revolution, the curriculum design committee (led by some top rate physics educators) decided to rewrite as a=sigma-F/m, because this emphasizes that acceleration is usually the quantity that is the result of the various forces.  I had some discussions with a big group about whether and why that made sense or didn't make sense, and how that change should be explained.  On the equation sheet now, it reads a=sigma-F/m = Fnet/m.  I don't know, but I suspect, that this dual notation is to accommodate the two dueling camps of physics teachers: those who prefer students to write using vector notation "Fn + (-mg) = ma" because down is the negative direction, getting a negative acceleration if the acceleration is downward; and those who prefer students to not use negative signs but just magnitudes of forces, writing either "Fn-mg=ma" or "mg-Fn=ma" starting with the direction of acceleration.  The former is sigma-F=ma, the latter is Fnet=ma.

Gardner Friedlander says: He quoted several sources from the last 50 years that use all three versions of what should be set equal to ma: F, Fnet, sigma-F.  

His thought - which I agree with, now - is that the difference is probably not so much a historical trend, but rather the intended audience.  Julius Sumner Miller and Paul Hewitt were aiming at a general, non-mathematical audience trying to understand physical concepts, so used just F.  Mathematically based courses preparing students for continuation in the physical sciences used sigma-F.  And those aiming for an in-between audience, like AP Physics B, AP Physics 1, high school honors courses - they tended to go for the in-between notation of Fnet.

And in a final twist... I've started changing my notation to "netF = ma".  I've noticed for years that Fnet is confused with Fn for normal force; and that it's a misconception that "Fnet" is a separate force like the friction force or a tension that should be on a free body diagram.  By using the language of unbalanced and balanced forces (as the Physics Classroom does), the net force is just the unbalanced force; so netF emphasizes something different from Fnet.

19 October 2020

Are Kepler's Laws Covered in AP Physics 1?

Kepler's laws are never a starting point for an AP Physics 1 question.  

That said... situations involving gravitation and planetary orbits are part of Physics 1.  AP Physics 1 does include circular orbits, from which one could in principle do the work that derives Kepler's T^2-r^3 law.  AP Physics 1 does include angular momentum conservation, from which one can understand the same consequences as Kepler's equal areas/equal times law.  There's no need to understand elliptical orbits, just circular orbits.  

Physics C includes some questions where the best approach is to cite one of Kepler's laws and logic from there.  Physics 1 does not - the approach to every planet problem starts with Newton's law of gravitation or conservation of angular momentum.

11 October 2020

The physics behind a football spiral

I was forwarded this article (wsj subscription required) in which author Jason Gay issues a warning: SPORTSWRITER DOING PHYSICS!  Well, I've done a lot of sportswriting and I've done a lot of physics.  I give Gay enormous credit, because his physics explanation was crystal clear to me.

Gay explains a recent paper in the American Journal of Physics, in which the authors show that gravity by itself is not sufficient to cause a football to spiral in the direction of its motion throughout its flight - conservation of angular momentum suggests that the initial angular momentum can't change without an external torque (which gravity does not supply).  The football's path should bend in a parabola... but the point of the ball should always point in the same direction.  Then air resistance should cause the ball to rotate end-over-end, "like a duck".  Presumably he means a duck in flight who suddenly experiences cardiac arrest, but I won't quibble with sportswriters' analogies.

This paper's authors - Richard Price, William Moss, and TJ Gay - show that contrary to previous models, torque provided by air resistance continually changes direction.  This changes the direction of the angular momentum such that the spiral always points tangent to the football's path.  Cool.  That makes sense.

Now, I'm an experimentalist... my next step would be to hire Patrick Mahomes (the reigning Super Bowl MVP, quarterback for the Kansas City Chiefs) to attempt to throw a spiral in NASA's enormous vacuum building.*  If the paper is right, then even Patrick Mahomes should not be able to throw a proper spiral.  Rather, though the football should spiral, the nose of the ball should continue to point in whatever sorta-upward direction it was spinning when the ball was released.  Without air, there'd be no threat of a dead-duck motion, but the spiral shouldn't gracefully arc throughout the flight as in the NFL Films films.

* Or perhaps on the moon.

Anyone want to write this grant?

04 October 2020

Mail Time: I love the AP Physics 1 Workbook. Why isn't there a physics 2 or physics c workbook?

If you teach AP Physics 1, I hope you've discovered the Workbook.  It's great.  But, the question was asked, why only for Physics 1?  Why not provide the same sort of scaffolded, sequential, task-modeling exercises for all physics courses?

There's not a Physics 2 or C workbook simply because of the necessary person-hours - and high-end expert person-hours at that - required to produce the book!  Amy Johnson, who's as expert as you'll meet, spent a truly ridiculous amount of time spearheading that project (and writing much of it herself).  

Then, AP Physics 1 was prioritized because that's the largest and hardest course, the one where it's rather commonplace for a school to tell a biology teacher "oh, you are certified in science, you can teach AP Physics 1, good luck."  I've met so many of these unfortunate folks.  They often become outstanding physics teachers!  But in those first couple of years, they need an anchor.  

The workbook can be that anchor.  While it's in no way good practice, it is nevertheless possible for an overwhelmed and inexperienced teacher to do nothing but assign the workbook, page by page... and their students would have a fighting chance of success in the course.  Then the teacher can build on that foundation in future years.  

And finally, P1 is the most misunderstood AP course.  "1 - Algebra-Based" conveys a sense of simplicity to nonexperts, such that administrators and parents and amateur college counselors routinely push weak students into this course, expecting an easy STEM AP credit - yet P1 is, statistically, the most difficult AP course of all.  Teachers don't always know better, either: so many assign Giancoli calculational problems, teach nothing beyond plug-n-chug, then are surprised and sour-grapes-y when few students pass the exam.  

The workbook is there to demonstrate the kinds of verbal responses that will be necessary on the exam; and to guide students (and teachers) toward building the necessary communication skills.  For strong students, AP C can be picked up from textbooks and Khan Academy-style videos.  Physics 1 isn't so easily mastered, even by top-rate students.

Hope that helps explain... Really, the physics folks involved with the College Board are trying to help everyone.  They ain't perfect, and they can't please everyone, but they're trying!  :-)

03 September 2020

position-time and velocity-time graph introductory exercise - simulation for students who aren't in class.

Motion graph simulation by Milo Jacobs

On the very first day of kinematics, I introduce position-time graphs via this come-and-show-me exercise.  Each student gets a different position-time graph, and is asked to use position-time graph facts to justify their answer to two questions:

1. Is the cart moving toward or away from the motion detector?

2. Is the cart speeding up, slowing down, or moving with constant speed?

Then, once I approve their predictions, the student heads to the back of the lab where I have carts and tracks.  They have to choose to use either a regular pasco cart, or one of them motorized bulldozers.  They have to choose how or if to incline the track.  They have to choose where to put the sonic motion detector that makes the position-time graph.  

Finally, they bring me the graph they made.  (I use the detectors that work via bluetooth with the graphical analysis app on a phone; if you're using a labquest or labpro, I just have students take a picture of the graph and show me.)  I check that the graph looks correct AND represents at least one second's worth of motion.  Then I give the student a different graph to go through the process again.

Once the class gets the idea that they are required to begin each response with a fact written verbatim from the fact sheet, they figure out very quickly how to do these exercises.  We can move on to velocity-time facts and exercises very quickly.  

But what if you have students who aren't in your lab?  How can they do this laboratory exercise from home?

I've got a simulation for you. It's certainly not the same as being in lab!  There's a huge difference between clicking buttons on a computer, and actually physically futzing with a track, cart, and motion detector.  Yet, a simulation is better than nothing at all, better than saying "okay, imagine the motion..."

There's no shortage of motion graph simulations around.  But none of them did what I wanted - I want the ability to use EITHER a motorized cart OR a free-wheeling cart.  I want to be able to place the motion detector anywhere I want.  I want to be able to tilt the track in any direction.  And I want to be able to choose whether to give the cart a quick shove, or to just let the cart go from rest.

This summer my family's computer programming department - that is, my son Milo - was stuck at home with no obligations.  I offered him a job programming the exact simulation I was looking for.

Take a look at Milo's simulation here - a screenshot is at the top of the post.  You adjust the track angle, click confirm; click a position for the detector; then click to push, release, or use the motor cart.  The simulation shows the cart's motion, and simultaneously shows the position-time and velocity-time graphs develop - exactly as if you were using a labquest in the classroom!

Then there's a link to download the graphs if you want an easy way to get a .png file to submit electronically.  Here's what they can look like:

I used this simulation this summer in my institutes.  It works - it's not the same as being in lab, but it's as good as I've found for teaching remotely.  Try it, and let me know how it works!

29 August 2020

Respecting your audience, and Rule 1 of teaching

Rule 1: Never condescend, nor even give the appearance of condescension.

I've had to watch a bunch of videos and zoom sessions already, with more to come, as we prepare for the school year.  These have been / probably will be valuable, in the sense of promoting discussion among faculty, of communicating necessary information to students and faculty.  So far, sessions have been respectful of my time - no one is reading powerpoints at the attendees, for example.  It's been good.

Today's presenter - who turned out to be pretty danged awesome - nevertheless turned me off from the beginning.  

While people were joining the zoom session, the presenter had already shared his screen which included five bullet points:

  • Try to be as present as possible
  • Remove distractions if possible
  • Get a beverage
  • Get note-taking items
  • Prepare to participate
Wait, Greg, aren't these reasonable cultural expectations for attendees at a zoom presentation?

Of course they are.  That's not the issue.

By telling us that which any professional educator should already know, the presenter communicated to me a sense of distrust.  "I know you naughty teachers will not pay appropriate attention to our meeting..."  

The presenter further communicated a hubris about their own power and influence.  "...but if I give you specific rules about how I expect you to pay attention, then presto, you teachers will follow these rules and pay attention.  Perhaps you just authentically didn't know that you should pay attention in a meeting, so I'm relieving you of your ignorance.  Or, perhaps you will sigh and say 'aww, man, I was planning on playing Candy Crush for the next few hours, but dammit, these are the rules, so I'd better go get my note-taking items."

Greg, you talk about culture building... doesn't culture building start with a foundation of what the interaction rules are?

Culture does begin with a foundation of shared expectations.  For our boarding students, most of whom are new or new-ish to living away from home with other teenagers, we start by discussing specifics about how a dorm community should function.  If I were teaching third grade, I would likely begin the year with an open discussion of how we should behave toward one another.  

However... culture building also considers the audience.  The audience for my physics classes are 14-18 year old students who have been in school for many, many years.  The audience for faculty meetings is a room of professional teachers - if they don't know that during a meeting they should "try to be as present as possible," then, well, they shouldn't have been hired in the first place.

Greg, you also know that in a typical meeting a quarter of the faculty are, in fact, playing candy crush.  Or possibly pac-man.

Quite possible.  There are but two effective remedies for this: (1) Make the meeting so worthwhile that most attendees forget about video games and focus, by choice.  Make the audience want to be attentive.  (2) Find one or two people who are blatantly disengaged, and have a conversation with them.  Do they want to be a part of the faculty, or not?  

These are the same remedies, by the way, that I recommend if you have similar issues with students during your class. 

Unfortunately, today's presenter chose ineffective and maddening option (3): give the audience perky-toned yet condescending "rules" for the meeting.  A commonly-used equivalent is the all-faculty email reminding everyone how meetings are important and we should make an effort to focus.  The candy crush players don't change their behavior in response to option (3).  

Rules do nothing but irritate the professionals.  And that was our first impression of the presenter.  He had a mountain to climb to win me over.

Now, I'll be fair to this presenter - he did win me over.  His 90-minute meeting was fantastic, to the extent that when he stopped I said "oh, wow, that was short, he could've gone on longer." How I feel at the end is a major way that I judge any performance.  So this presenter passed with flying colors.

But he could have made things so much easier on himself.  And, how many other folks in the meeting did he lose just by implicitly questioning their professionalism?


04 August 2020

What equipment do I use to record my physics shows?

I've fielded the question enough times that it's worth a blog post.  In my AP Live videos in spring 2020, in the upcoming AP Daily videos, in my summer institutes... what equipment did I use for recording?

The main camera is a logitech "Carl Zeiss Tessar" model mounted on a tripod facing my demonstration table.  It helps that my whiteboard is low-glare... the reflection from the windows and lights is not bad.  I "enable high definition" in the video settings in Zoom.

I also have a document camera on the edge of the demonstration table.  To switch between cameras, in zoom I say share screen --> advanced --> content from second camera.  This brings up the document camera.  But then I see a button in the top left of my screen to "switch camera" - click that, and I seamlessly switch from the doc cam to the logitech.

As for audio... both my doc cam and the logitech include a microphone, and I can put audio to the speakers in my classroom.  But the sound quality that way isn't great.  So I use a plantronics headset, as pictured in the top right.  The sound is as good as I've ever heard!  The only disadvantage is that I am wired to a USB port.  I've several times stepped on the wire and accidentally disconnected.

Speaking of USB ports... that's three items that need ports.  I had to get a multi-port hub.  And it's important that my internet connection is hardwired to the desktop computer I'm using - otherwise things can get annoyingly slow on Zoom.  

I'll give a shoutout here to Woodberry Forest's academic technology guru Cronin Warmack.  He's been extraordinarily helpful offering whatever technology I need, and helping to make sure it actually works.  All spring and all summer, I had working computer, internet, printers, copiers, network storage, software applications... that's not a trivial thing.  

At my previous school, the head IT guy was offended that I requested the technology that I had been promised at my interview.  While that school's administrators always had working equipment, it seemed beneath this guy even to respond to a mere teacher's concern, let alone to actually act upon it.  And I am aware that many schools operate exactly this way regarding the most basic tech.

Not here... Cronin (and Aiden and Jason) have kept Woodberry running online.  I appreciate their work.

23 July 2020

As we have to cut our courses to the essentials... what are the essentials?

I've been asked a number of times about what to do if you have to teach a portion of the year online.  What are the most essential elements of a conceptual physics course?

First and most important, please read this post about the purpose of teaching physics.

But even once you share my goals about purpose and experience and culture, the question remains - what topics are you retaining?  Which are you cutting?  What are you doing with your class when you're exclusively online?

In all seriousness - at levels below AP - I recommend that if you have to be online to start the year, you just play with your class.  Do Sporcle, Dungeons & Dragons, intellectual games like Charty Party or Codenames, Crayon Physics Deluxe... whatever a nerd like you (and me!) would do with your family or friends on a Saturday night.  These are the sorts of things that my college friends did on weekends; come to the AP Physics readers' lounge and you'll see these things, too.  That's not physics, you say.  It's not.  It's relationship building.  And when you get back to actual school, your students will be way on board with studying physics because you didn't pile on the work when they were stuck at home without same-age companions for the seventh month in a row.  And physics taught live in person can feel like a video game, anyway.

But what topics do you cut out to trim to the essentials?  What do you try to "teach remotely" if you have to?

On one hand, building skills in conceptual physics is topic-agnostic.  You can teach effectively, you can give students an excellent first-year experience, with any combination of topics.  I recommend picking topics that you have equipment available for.

Then, for this oddball year, I recommend starting with non-cumulative topics.  We've done this for years for our 9th grade class - at boarding school, a 14 year old is often so overwhelmed living life on their own in an intense new culture that they aren't paying so much attention to their studies.  We do serious physics in the fall trimester.  However, a student who falls behind or doesn't quickly adapt to high school academic life isn't lost.  Quite frequently, a student does poorly on our Thanksgiving exam, but comes back in December with new life - more confidence, more experience living in our community, more willingness to study.  And they just jump right in, because we're starting from scratch with motion.  (Had we started off with motion and force, they'd be totally left in the dust when we move on to momentum and energy!  But if you don't get circuits, you can still do fine in motion and force.  These topics don't really build off one another.)

Originally when we designed our course, conceptual physics looked like this:

1. optics
     a. reflection/refraction
     b. lenses
     c. mirrors
2. waves
     a. v=λf
     b. sound and light
     c. dopper/resonance/diffraction
3. circuits
     a. Ohm's Law
     b. resistors in series and parallel
     c. power in bulbs
4. motion
     a. position-time graphs
     b. velocity-time graphs and acceleration
     c. equations describing motion
5. force
     a. direction of force and motion
     b. Newton's second law
     c. Newton's third law
6. motion and force in two dimensions
     a. adding force vectors in two dimensions
     b. Newton's second law in two dimensions
     c. projectile motion
7. impulse/momentum
     a. conservation of momentum in collisions
     b. impulse-momentum theorem
8. energy
     a. forms of energy; the energy bar chart
     b. making predictions using energy bar charts
9. harmonic motion
     a. objects on springs
     b. pendulums

About four years ago, our school changed schedules; we lost a small percentage of our time with students.  So, we had to cut - we cut out mirrors, we cut out doppler/resonance/diffraction, we cut out harmonic motion.

It's not so possible to make careful plans this year.  The world was different a month ago, and certainly three months ago.  The world will be different again in October and January, in ways we can not predict. 

So what should you cut?  I certainly recommend starting with non-cumulative stuff this year, or planning the non-cumulative stuff for the most uncertain times.  If you must be entirely online and you must do some sort of real physics, do optics and waves (and circuits, if you have to!)  Then when you return, dive into motion using your carts and tracks and detectors.

My boarding school is planning a college-style schedule... that is, we're all coming back to campus, but we're leaving at Thanksgiving and most likely not returning until January.  So... I'm cutting the circuits unit, and moving the more difficult and more experimentally intense motion unit to October.  If we can come back in December, I'll cover circuits then; more likely, we can either try to study circuits with the excellent Phet simulation; or, I can cut it entirely and play games with my class online in December.  I'll see how things go.