How I'm using the 2020 released AP problems for exam review: Identical Situations

The 2020 digital AP physics exams were all digital, all at home.  That meant the development committees had to go many extra miles to produce exams which made cheating nigh-on impossible in the moment.  For the Physics 1 exam, at least, that meant on one hand a huge variety of problems.  Yay!*

*Note that I am emphatically NOT on any development committee.  I don't think those folks ever said "yay" as they desperately put these exams together under significant time pressure last spring.  But I say yay, and send them my deep thanks.

Even better, many of the 2020 exam problems came in pairs or triplets referencing identical situations, but with subtly different questions asked about the situation.

Why are these sibling exam problems "even better" than a bunch of brand-new problems?  Because these have led to some of the most valuable exam review activities I've ever experienced.

Novice physics students tend to see every problem as completely new, completely impenetrable at first.  We know that they should start with a fact of physics or an applicable equation, and then they should figure out how those facts and equations apply to the specific situation.  They too often think they should know the answer right away, and that if they don't they're stupid.   

I remember a quote from a guest speaker at one of our opening faculty meetings.  He referenced research about how children learn to read: they learn best when they repeatedly read about topics they're familiar with.  A kid likes dinosaurs? They should read about dinosaurs.  Again and again and again.  

So this spring I've had my students do problems involving situations they're already familiar with - by assigning the triplets of 2020 exam problems that reference the same situation.  They do the first version for homework, getting some right and some wrong.  Then they do the second version for homework.  Suddenly, since they are already familiar with the situation, they are paying attention to the physics principles in play!  I can see the greater confidence on night 2.

Then on the third day, I've given the last version as an in-class quiz, which we grade to the official rubric.  I got the highest quiz scores in recent memory!  Now, obviously, there wasn't the issue of coming to grips with a new situation.  But that's the point!  My students, for the first time, internalized the idea that every new problem can be solved by applying physics principles.  Any mistakes were due to physics misconceptions, not issues with reading comprehension or personal confidence.

How do I get these problems, Greg?  They are available on AP classroom.  Like everything in that platform, it requires an application of black magic to accomplish what you want to do.  I created a new quiz; then I searched for problems worth 10 points.  See, all "normal" AP physics 1 problems are either 7 or 12 points.  But in 2020, the paragraph problems were 10 points, the qualitative-quantitative translation problems were 15 points.  Searching by point value got me just the 2020 free response.  Then, I looked for identical descriptors, put them into a single "quiz", and assigned the quiz in hard copy.  (The interface lets you print out the answer sheets.)  Yes, that's a lot of work to get the problems you need.  Nothing in AP classroom is easy or intuitive.  But these problems are there... and they are worth using your reserved black magic spells!

Where did the 2020 AP Live videos go? (Don't worry, here's how to access)

Hi, I'm Greg.  You may remember me from such films as the 2020 AP Live video series.  My friend Josh Beck and I were live on the College Board youtube channel every weekday from 12:00-1:00,* from late March to early May.

*P.M.

The College Board is doing AP Live again!  Josh and Kristin Gonzalez-Vega will be running similar live sessions this year, starting April 19.  They are outstanding physics teachers; I recommend their shows highly.

That said, I've been asked a number of times, are the videos from 2020 still available?  A number of teachers (including me!) have been using pieces of these in their classes as content introduction or review.  They've been been up on the College Board youtube channel all year, until last week.

The College Board took the 2020 videos down for very good reason.  Josh and I were laser-focused on preparing students for the 2020 digital at-home exams.  Those were completely different in structure from any exam given before, and from almost any exam that will ever be given again.  The last thing the College Board wants is for well-meaning students to have these videos come up in a search, miss the whole "2020" thing, and panic about digital testing.

Thing is, physics content is physics content, pandemic or no!  That's why teachers will want to access these videos.  Josh and I did uncountable quantitative demonstrations.  We set up released AP Physics 1 problems as experiments in the laboratory.  We explained how the readers evaluated released free response questions, with special attention to common misconceptions.  These videos have been a valuable resource to a lot of us.

If you do end up watching, please remember that we did these shows live.  There will be small mistakes.  There will be experiments that fail the first time, and we didn't, couldn't, edit.  It was live.  I love live performance, and I know that a huge audience of students and teachers loved it, too!  Just know you're not going to be getting anything polished.  Go see Pivot Interactives if you want that.  :-)

  

(My own recent use of these videos: I've been assigning some old released AP questions for homework, then giving a quiz about the problems the next day in class.  If a student does poorly, I don't have them come in to redo the question - I just have them watch the video of Josh or me discussing that very same problem!  I encourage them to watch on double-speed, but I make them watch.)

How do we access the videos, then?  The videos are still in existence on youtube, but are not searchable.  You have to know the link.

I went in my creepy youtube search history and dug out as many old links as I could find.  I'm not comfortable posting those links here; but if you'll email me, I'll send you a file with those links, which we should all feel free to share teacher-to-teacher.  Or even with our students.  

My understanding of the College Board's position is that they're happy for people to watch Josh's and my time capsule from spring 2020, as long as the watchers know exactly what they're seeing, and as long as the watchers seek us out explicitly.  The information in our 2020 shows is NOT CURRENT.  But it's still interesting and fun.

Mail time: angular impulse for a block-pulley system

A physics phriend discovered a question about a massive pulley.  Look at the diagram to the right.  The hanging block of mass m is allowed to fall from rest.  What is the change in the angular momentum of the block-pulley system during a time t after release?

The answer given by the problem author was mgRt.

Is that right? Doesn't that assume the tension in the rope is equal to the weight of the block?  I know the tension is less then mg, because the forces on the block must be unbalanced.  What am I missing?

I love this question, and I love this question about the question!  The change in angular momentum is equal to the unbalanced external torque on the system multiplied by time interval - that's the impulse-momentum theorem applied to angular rather than linear momentum.

So, what external forces act on the pulley-block system?  The pulley itself experiences a contact force from its support, and the force of the earth.  (These are balanced.)  The block experiences the force of the earth.  Since nothing balances the gravitational force of the earth on the block, the gravitational force is the net unbalanced force.

What about the tension, then?!?  The tension is INTERNAL to the block-pulley system.  Yes, sure, I didn't mention that the rope pulls down on the pulley and up on the block, because that doesn't matter.  I'm looking for EXTERNAL forces only.  

So the unbalanced external force on the system is just mg.  This force provides a torque of mgR.  And multiply by time interval t to satisfy the impulse momentum theorem, and Bob's your uncle!

Hope this helps...  we're never assuming anything at all about the tension, equal to or not equal to mg.