Live Physics Time on Twitter: Monday May 31 2021, 7 pm eastern - it worked!

Postscript: Thank you to the dozens who listened as I discussed the 2021 AP Physics 1 exam.  The stream worked.  Now, I'll use this to call Saturday's Portland Thorns vs. the Fellowship of the Ring match; and, I'll use this to do future spontaneous Live Physics Times.  Send in your questions and thoughts!  If I don't have time to write up a blog post, I may do a Live Physics Time while I walk the dogs some morning!

Hey, all!  I want to try out the Twitter live broadcast function.  So tonight at 7 pm (eastern), I'm going to, um, try it out.

Look on my twitter feed @gregcjacobs.  If all works as planned, you should be able to hear me talking live about the 2021 AP Physics exams, about conceptual physics, and about whatever people want to know about.  Have a question?  Drop it on twitter during the broadcast.

This is all a trial!  If it works well, I have two things planned for the feature:

(1) The Central Virginia Portland Thorns Fan Club will use it to broadcast their watch parties live; and

(2) I can do more Live Physics Time in the fall, maybe even from my classroom.  

We'll see - the possibilities are numerous once I can get this broadcast feature working.  Tonight, though, is deliberately last-minute.  I'm hoping that a few dedicated physics teachers or students tune in.  I'm hoping that a small group of us have a good time together, for 20 minutes or so (or longer if there's an audience that wants to keep going!).  And I'm hoping that the group is small and friendly enough that when something doesn't work right, it's okay.  :-)

- GCJ

Mail Time: How will the 5 Steps book be updated for 2021-22 with changing AP1 course content?

I was wondering if you were going to update your 5 steps book for the 2021 – 2022 school year since the course content has changed? And when would that become available?

We are updating for the 2022 edition.  Thing is, since the announcement that waves/electricity would be incorporated into AP Physics 2 came after the books were in the proof stage, we're keeping the waves/electricity material in the Physics 1 book.  We're using shading, graphics, and repeated exhortations to indicate that these are no longer P1 topics, but are now part of P2.  This way P2 students will still have the resources if they want them.  

Then for the 2022-23 school year edition, we will completely move the former P1 waves and electricity topics from the P1 book to the P2 book.

The new book is scheduled to release in early August.  BN.com says August 6.  You can pre-order now!

The velocity of a system's center of mass doesn't change in a collision...

After I posted my 2021 AP Physics 1 solutions, I got an important question about problem 3, the qualitative-quantitative translation.  The problem asked students to estimate the speed of the center of mass in two separate collisions.  The "estimate based on a thought experiment" part is very similar to 2019 problem 2, which asked a very similar question about the acceleration of a modified Atwood machine.

The question, though, was about the center of mass speed:

The velocity V_cm is the velocity of the center of mass for the two-object system.  I agree that the velocity of object 1 would change or not change depending on its relative mass to object 2, but because there are no external forces on the system, I think there would not be a change in velocity of that system, thus V_cm will remain unchanged.  

So, this physics is 100% correct - *within each trial*, the center of mass speed does not change.  That's why we can say that whatever the combined objects' speed is after the collision, that's the speed of the center of mass throughout.

The actual question asks about different situations, though!  In the first situation, Edna the hippopotamus is moving, and she catches a peanut.  The center of mass of the Edna-peanut system doesn't change speeds during this trial - right.  So the center of mass is moving at whatever speed Edna moves with the peanut after collision.  That's pretty darned close to Edna's initial speed.

In the SECOND situation, Bertha the elephant is stationary and catches a peanut.  It's correct that the speed of the Bertha-peanut system doesn't change after collision.  The speed of the peanut does change, though - the peanut slows down when Bertha catches it.  After the catch, Bertha and the peanut are hardly moving at all.  So the center of mass speed is very small!

2021 AP Physics 2 exam - solutions

The free response questions from the May 7, 2021 administration of the AP Physics 2 exam have been released.  I had fun crafting my own solutions.

Here are my solutions to the 2021 AP Physics 2 exam.  (My solutions to the more common AP Physics 1 exam are available to teachers at this post.)  This solutions file is accessible to TEACHERS ONLY through the "pretty good physics - secure" site.  If you're a student and want access, ask your teacher to join pgp-secure using the instructions on the site.  There's no point in pretending to be a teacher - the folks in charge check membership requests carefully, and they will take away your birthday if you're not actually a teacher.  And I'm sorry, I am not allowed to send these to students or to post publicly.  The College Board's lawyers would take away even more than just my birthday.

The first problem gave a nice combination of macroscopic and microscopic questions about thermodynamics.  My own students will find the macroscopic parts simple, but may not be great on describing the microscopic issues in part (b)ii.  I'm okay with that.  There's so much material in physics 2 that I've gotta make some choices about how deep to go.  If my students get 8/10 because they had no clue on one wee part of one problem, I'm okay with that.  

The tradeoff is, much of the rest of this exam should be in my class's wheelhouse.  This year, I was very much time-limited for AP 2.  You may have made different choices about what to focus on, and that's totally fine!  If your school was in person all year, maybe you didn't have to make choices at all.  Play to your own personal strengths.  It's always better for someone to know fewer topics very well, than to know many topics sorta halfway.

The second question seemed like it was also about thermodynamics, but at its heart was about experimental skills.  The main question just asked about how to measure quantities experimentally.  Then students needed an understanding of static fluids.  Only the very last part required using any actual knowledge of theoretical thermodynamic relationships.  

I loved the qualitative-quantitative translation question, which focused primarily on reading and describing graphs based on an understanding of electromagnetic relationships, rather than on manipulating equations.  

The last question beautifully puts energy and momentum conservation in the context of subatomic particles.  Many years ago when I first heard about the "big ideas" of AP Physics 1 and 2, I assumed that particle physics like this would be in AP Physics 1!  The only conceptual difference between this problem and a physics 1 problem is understanding new ways of energy transformation and manifestation - mass can convert to energy, and the energy of a photon isn't determined by its speed.  It's still the "big idea" of the conservation law for energy.

As always, I guarantee I got a 5, but not that I got a perfect score.  Send me your thoughts! 

2021 AP Physics 1 exam - solutions

Ah, it's good to be able to download the questions 48 hours after the exam again.  It's good to be back to five total questions, two long and three short.  We're back to something approaching normal in the AP Physics 1 world!

Big thanks to the College Board for the format of the test question download (available here).  Note that there's space for students to answer each question!  For the first time on 25 years of teaching AP Physics, I don't have to spend time cutting and pasting.

Anyway.  Here are my solutions to the exam.  This file is accessible to TEACHERS ONLY through the "pretty good physics - secure" site.  If you're a student and want access, ask your teacher to join pgp-secure using the instructions on the site.  There's no point in pretending to be a teacher - the folks in charge check membership requests carefully, and they will take away your birthday if you're not actually a teacher.  

I loved these questions, as has been typical for a while.  The AP Physics 1 development committee is in a nice groove in which the problems are predictable in style but unpredictable in content.  Awesome.

For number 1, it's much easier if you've memorized the "range equation" for kinematics.  My students haven't; but no worries, it's still doable, just requiring an extra step of algebraic kinematics.  No trig identities necessary for credit!

The experimental question required essentially no physics content knowledge, only experimental skills.  (Graphical analysis is an experimental skill!)  Fantastic!  No free points for writing or deriving equations, just for doing and analyzing results from a lab.  Cool. 

The paragraph question was clever and - for those who understand energy conservation well - straightforward.  

For number 5, the wording is important!  They're not asking for the AP Physics C-level three-body problem with a massive pulley.  They want the net torque on the objects-pulley system.  So the only external force is that of the earth.  You can use mg*lever arm to find each torque.  Then parts b and c are similar to the problem I did in my very first AP live show in 2020, back when Edna wasn't even wearing her bow!

As always, I guarantee I got a 5, but not that I got a perfect score.  Send me your thoughts!

AP Physics 2 fundamentals "quizzes" - great review exercise in an unusual year.

 Hi, all!  We're winding down a weird year.  This year my AP Physics 2 class was an intensive half-year course that started Feb. 4... the day after we were all sent home due to the plague.  We came back into the classroom on March 24.  Good luck teaching AP2 on that schedule!

The folks in my class all had a very strong AP Physics 1 background - they knew how to learn physics already.  Thus, we accomplished a lot by doing problem solving practice based on fact sheets, with demos and quick virtual "experiments" thrown in.  When everyone returned to the lab, I could show the real live physical situations, and students had the context to understand quickly.

Once we had discussed the basics of most units, I began giving a fundamentals "quiz" every day.  These have never been for a grade!  I'd have gotten terrible pushback if they were.  In fact, this whole course has been not for a grade, but for AP exam prep alone.  Sure, my juniors work diligently every day while the seniors play paper football on the lab tables for a larger percentage of class each day.  Right now, this year especially, I don't care.  Even the seniors take the quizzes seriously.  Even the seniors are making progress, though slow progress.  Everyone is getting better every day.  

It's these quizzes that are the best review.  Simply the experience of taking the quiz and then getting instant feedback helps.  There's no pressure to perform, there's no shame for the student who gets 3/10.  But everyone is still a wee bit competitive!  When a weak student was one point short of my best student's score, he celebrated as if he had won a baseball game!  

I don't know if all my students will pass.  Those hard working juniors - they'll get 4s and 5s.  Those seniors - they've got a shot.  And they have a much better shot at passing the AP exam now that they've taken fundamentals quizzes every class day for weeks.

How do you create these quizzes?  I have created a draft fact sheet for AP Physics 2.  I'm not ready to publish that yet, because it's still very rough.  But the process here is the same as for every course: I copy the entire fact sheet, and paste into the random.org list generator.  Then I riff off of each fact.  

Sometimes I ask the fact straight up: "A positive charge is forced ___ the direction of an electric field."  Sometimes I put the fact in a specific context: "An electric field points right. What is the direction of the electric force on a positron in this field?"  And sometime, I get a bit tricksy* by asking something like "what happens to a positron in this field whose initial velocity is to the left?"

There are no trick questions in physics - in fact, questions like "A car moves at constant speed 4 x 10^9 m/s, what is the car's acceleration?" are explicitly prohibited by the College Board!  An answer choice, let alone the correct answer, may never "deny the stem."  The exam will never expect a student to respond, "hah, that's impossible, nothing can move faster than the speed of light, so your question itself is bunk!"  There are certainly tricky questions that require careful multi-step reasoning; but never trick questions.

The resulting quizzes provide cumulative review that hits every topic on the exam several times over many weeks.  Even when students get answers wrong, they learn from their mistakes.

Given the shortage of time and student energy, I'm choosing to focus on fundamentals.  These folks have strong problem solving skills.  But those skills are useless if they don't know when to use F = qE as opposed to F = qvB.

Would you like to try these fundamentals quizzes?  Be my guest.  They're in this folder here.  I give a strict four minute time limit for each, because if you don't know these right away, you ain't gonna figure them out by thinking for a while!