Simulation and experimental evidence for 2021 AP Physics 1 problem 4 - block and cylinder on incline

About half of the videos in my 2020 AP Live series discuss released AP Physics 1 free response problems, including experimental evidence for the solutions.  I use these videos regularly during exam review time: students solve a problem, then if they don't do well, they are tasked with watching the corresponding video.  

I need to start updating with experiments covering the released problems since 2020.  And here's that start.

Last summer, my son Milo asked me to partner with him to enter a math/science video contest.*  I had been grading the paragraph-response problem, number 4 on the 2021 AP Physics 1 exam.  A student from Georgia, Widener Norris, had pointed me to the deeper meaning behind the energy bar chart for an object rolling down an incline.  Milo coded a simulation to produce energy bar charts and energy-vs.-position graphs for sliding blocks as well as rolling objects.  

* Our video earned honorable mention.

And so I did this 12 minute live show about the problem.  The show references Milo's simulation, which I highly recommend for standalone exploration about the cylinder/block situation.  You can change the incline angle, coefficient of friction, and type of object; press play, and you'll see the energy bar chart and energy-vs.-position graphs develop, frame by frame if you choose.  

The video shows, in real time and in slow motion, a block and cylinder sliding down an actual incline such that they reach the bottom with the same speed - just as the AP question postulates.  And then I discuss an unusual method of experimentally determining the rotational inertia of my pet hippopotamus Edna as she rolls gleefully down an incline.

Imprinting, and teaching spring energy before spring force

The first day on which you introduce a new topic imprints on your students’ brains, the same way a duckling imprints on its mama.  If from birth you replace mama duck with, say, a dog, then the duckling follows the dog around in an adorable manner.  

Similarly, students imprint on your first lesson in a new topic.  Though less adorably.

In electrostatics, if your first lesson is about Coulomb’s Law, then F = kQQ/d^2 becomes the starting point for every single problem, even those that don’t involve forces or point charges.  So instead, I make the first lesson a conceptual introduction to the meaning of an electric field using F = qE (with PE = qV following up on day 2).  The force between point charges follows a week or two later, once the concept of electric field in general has been well established.

In energy, if your first lesson is about work, then every energy problem starts with W = Fd, even those that don’t involve a steady force (or any distance to speak of).  So instead, I make the first lesson a conceptual introduction to energy bar charts.  Work becomes the thingamajigger that changes mechanical energy in a bar chart.  Later, once students are well used to creating, annotating, and using bar charts, I mention the fact that work is the area under a force vs. distance graph.  That becomes W = Fd if a force is steady.  

I’m going to try to fight another imprint that my AP reading colleague Peggy Ankney brought to my attention.  Haven’t we all been frustrated that every spring problem starts with F = kx?  Even when the problem asks for a speed rather than a force or distance, even when the problem explicitly discusses the potential energy of an object-spring system?  

This coming school year, I’m going to invert my first-time approach to springs.  Instead of a laboratory exercise with F = kx, I’m going to start with lab exercises using PE = ½kx^2.  I’ll introduce and use the formula for spring energy well before we ever discuss the force of a spring!  I want the default for my students to be to use an energy bar chart when they see a spring.  Students tend to consider force and motion methods before energy methods – because we *start* with force and motion.  Imprint again.

If I can get students habituated to the reflex that a spring implies use of energy methods, then hopefully (a) they’ll get most problems right because in fact a spring problem more often than not requires the use of energy, and (b) they’ll have an easier time recalibrating to the familiar concept of force than if they had to readjust once they’ve already grabbed the hammer that is F = ma.

Why does it take longer to grade AP exams remotely than in KC?

Cat sits on computer.

Dog barks to go out.

Cat demands breakfast.

Touch screen is slow.

Other dog becomes angry at cat for eyeing her bone.

Computer connects to wrong wireless router.

Cat barfs on kitchen floor.

Child demands lunch.

During the reading in Kansas City, the biggest distraction is trying to find the rare peanut M&Ms in the candy bowl.

A more interesting post - inspired by conversations in KC - will follow shortly.  :-)

In Kansas City Now...

It's been three long years in the wilderness, but the AP Physics readers are finally gathered in Kansas City.  We have 500(!) readers now - when I started in 1999, there were 79.  Point is, I don't see everyone every day!  If you're here and want to say hi, you can find me in the Loews lobby most nights after dinner; or Edna and I are grading the P1 non-operational exams in the room by the snacks.

Because I'm not on the operational exam this year, I can't discuss my particular questions or their rubrics.  But if you'll sign up for an AP Summer institute - the June 27-30 online institute through PWISTA has plenty of available spaces! - I'll talk you through the P1 rubrics this year, or any year.

I'm more and more able to articulate why I love the reading so much, other than the obvious professional development benefits.  Or maybe not obvious: The reason I can grade papers 5-10 times as fast as my colleagues at school is precisely because I've done so many years of boot camp at the AP reading.  We all learn something new to bring back to our classrooms.  After just a few days I already have ideas percolating.

The real reason I keep coming back is about culture and community.  So many places I've been - including jobs, including college and high school and middle school - have been full of people with social or professional agendas.  I've never, ever felt part of a wider community, never felt like I could sit at any lunch table and be truly and overtly welcomed.  

But here?  People know me.  They seek my input, and listen to it as I seek out and listen to theirs.  We argue about physics and rubrics, but disagreements don't lead to personal animosity.  Everyone here knows we are in a crucible together trying to grade all these 200,000 exams the right way, and we know the right way demands us being supportive, kind, and inclusive.  Importantly, those very few over the years who haven't done things the right way don't come back.  

The readers here are all very different.  They wear overalls, dresses, grungy t-shirts, dress shirts with slacks, yoga pants.  They speak with NPR-ready voices, and with thick drawls.  They teach at all sorts of universities and high schools.  It doesn't matter.  We all have a love of physics, and of physics teaching, in common.  We have a common goal of getting these exams graded (which gives everyone a point of idle conversation: not "what do you think about the thunderstorm last night," but "what problem are you on, and how's it going?").  

I'm thrilled to be back here.  I hope all you AP teachers out there will come join us at some point.  And I hope and wish that every community I am and will be part of could become as welcoming as this one.