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!