That's not how they checked the map

Trivia night last Thursday.  My team is down eight points to the couple with the beautiful dog at the next table.  Final question coming.  Category: African geography.  Twenty points on the line.  They get it, they win (but we win if we're right and they're wrong).  Here it comes.

Put these Mediterranean countries in order from east to west: Algeria, Egypt, Libya, Morocco, and Tunisia.

We can do this!  Some quiet discussion ensues at both tables.  We turn in our cards.  It's over, though we don't know the result.  Time for petting the dog, along with idle conversation.  We know we're probably toast - we tell the other team how impressed we are with their knowledge of television legal dramas, the category in which we fell way behind.  We discuss the final question.  "We think we got it," we said.  "But I'll bet you did too, so you're winning."

Remember, I grew up in the 1980s.  I remembered Gorbachev and Tears for Fears, both of which were useful on the night.

"I wasn't sure at first," said the woman on the leading team.  "But I checked when I was in the bathroom just now, and we were right!"

"Oh, that's very cool," I said,  "that the bar has a map of Africa hanging in the women's bathroom!"

Cue laughter as I realized what I said, and what she meant, and how old I must have seemed.

Visual Accelerometer: Noah Segal's (free!) iphone version

I've been a fan for decades of PASCO's visual accelerometer.  The linked post describes the demonstrations I use when introducing the meaning of the direction of acceleration.

Thing is, the visual accelerometer with the red and green LED display has been discontinued by PASCO.  They offer the Smart Cart vector display, which is great in that it does the same thing as the visual accelerometer AND it can be set to display force or velocity instead of acceleration.  But the display itself is $109 plus shipping, and it requires plugging into a $199 smart cart.  

How can you do the visual accelerometer demonstrations for free?

Noah Segal, on the PrettyGoodPhysics teacher message board, shared his solution: he programmed a website to display the output from the iphone's accelerometer exactly like the old visual accelerometer - right down to the red and blue colors.

Turn on the orientation lock on your iphone.  Then, go to the site indicated by the QR code at the top of the post.  You'll need to give the site permission to use your sensor.  Then hit "toggle mode" to see the 1-d red and green output.  Don't like the scale?  Zoom in and out with the + and - buttons.

Best of all, it's free.  Until your phone flies off a cart, hits the wall, and shatters.  Then it is a rather expensive toy.  But for now, just mount the phone securely!  :-)

Thanks, Noah.  

Mail Time: Gravitational potential energy in orbits

One of the Fundamentals Quiz questions in the leadup to the 2023 AP exams asked about gravitational potential energy in orbits: 

118. Satellite A is in low-earth orbit; satellite B orbits much farther from earth's surface.  Which has greater gravitational potential energy: the satellite A-earth system, or the satellite B-earth system?

A reader asks,

Can you explain why satellite B is greater.  Physics is not my strong suit and I understand why using Ug = mgh.  But when using Ug = - GMm / r , wouldn’t the gravitational potential energy be greater for A?  I’m a little confused.

Ah, this is the million dollar question in gravitation.  The negative sign is extra important here.  Energy is a scalar - the negative sign doesn't just indicate direction, as it does for force or acceleration.  A potential energy of -50 J is LESS than a potential energy of -25 J!  

So, for the satellite farther away, the denominator of your equation is greater, the whole fraction has a smaller absolute value... but the negative sign matters.  The negative sign ensures mathematically that objects farther away from each other have larger gravitational potential energy .

For AP Physics 1, I just write this last sentence as a fact - you almost never need to use this equation for gravitational potential energy.  (This will be true for the 2024 exam; for the 2025 exams and beyond, students may well need to use the negative sign fluently.  We'll see. Either way, I'd *start* by using the fact and ignoring the equation.)  For AP Physics C, use of the negative sign in the gravitational potential energy equation is an important concept mathematically as well as conceptually.

Breaking news: AP Physics updates, to take effect with the 2025 exam

The universe now has clarity about forthcoming changes to all four AP Physics exams.  I'm thoroughly impressed with the work being done by the College Board team and the development committees.  Here's what you need to know!

The changes I describe below will take effect with the 2025 exams.  The 2024 exams will be identical in form and content to those since 2015.

Let's start with content changes:

Physics 1 will now include fluids - just teach the current P2 fluids unit, or old Physics B material.  That's the major change.  Minor additions include the parallel axis theorem, the center-of-mass formula, and quantitative understanding of elliptical orbits.

Physics 2 will add in sound, specific heart, blackbody radiation, and a non-quantitative treatment of transient behavior in RC circuits.  Physics 2 removes special relativity and quantum wave functions (keeping E=mc^2).

Neither Physics C course will change its content significantly.  The new Course and Exam Description will include explicit references to some topics such as physical pendula which have been tested before but not named in old CEDs.

Now, what about the exam format changes?  These are rather significant.  The College Board representatives emphasize, as do I, that the style of questions on each exam are not changing much at all.  The format changes are for the purpose of clarity of expectation.  The CB wants students to know what sorts of questions to expect on each exam, such that they can worry less about reading directions and instead show their physics knowledge.  The CB further wants to be able to better differentiate the level of the 1/2 exams from the C exams.  A consistent format makes it easier to separate the skills of "being a good physicist" and "being a good test taker".

So there's no need to throw out the questions you've been using on your classroom tests!  You can cut out parts or add parts... but everything that's been given since 2015, and everything you can find on AP classroom as aligned to the exam, is still perfectly useful.

All four AP Physics exams will have the same format.  That format includes 50 multiple choice questions, each with four choices and a single correct answer; and 4 FRQs.  The timing will be 90 minutes for multiple choice, followed by 90 minutes of free response.  The equation sheets for all exams will be aligned.

The four free response questions on all exams will follow the same order and point distribution:

1. Mathematical Routines, 10 points.  This will include calculations and derivations; on the 1/2 exams especially there will likely be a part requiring multi-step verbal reasoning as well.

2. Translating Between Representations, 12 points.  Physics concepts can be communicated in many ways beyond merely equations and words.  This question will test students' ability to understand more than just one of these communication methods.

3. Experimental Design and Analysis, 10 points.  This will be very similar to the current questions on all exams that are posed in a laboratory setting.  It will likely include data collection techniques, even on the C exams.

4. Qualitative/Quantitative Translation, 8 points.  This is a shorter version of the style of question that has been on the 1/2 exams since 2015.  Fewer points allows the question to get straight to the point - describe a situation verbally, describe a situation with equations, and then show how the equations relate to the words.

That's all.  These changes are all made with the idea of transparency - if students knows exactly what to expect on the exam, then they can focus on showing their physics knowledge in each situation which is presented to them.

The College Board will be releasing three all-new practice tests in each course for teachers - they'll be available in 2024 after you pass the course audit.  Therefore, I'd recommend that you read these, and then cut and paste into the classroom tests that you're already using. 

The other exciting news, especially for P1 teachers, is that a new standard setting will happen for the 2025 exams.  That means a new set of comparisons between AP students and college students taking a similar exam.  And, that means re-evaluated cut scores to earn 5, 4, 3, etc.  The hope and likelihood is that more P1 students will be qualified for higher scores.  But stay tuned.  

Greetings from Physics Camp

Something like two dozen AP physics readers descended upon Gates Barbecue this evening on "dine out night", the night when the College Board was paying our restaurant tabs.  I had fun conversations on the walk there, during dinner, on the walk back, and in the Westin lobby where two different sets of physicists were playing two different card games.  

Regular blog followers know how much I love the AP reading. Grading papers all day for two weeks is intense, in the same way the football two-a-day practice week or band camp are intense.  The skills built for the season pay enormous dividends in the long term; and the camaraderie, the relationships created, can't be replicated in less intense venues.

I have lots of thoughts for future posts, including a discussion of the physics meaning of the colloquial word "faster", what the word "conservation" means in our students' minds, and the physical reasons for an experimental discrepancy.  But eight hours of brain work requires sleep sometimes... so I'll be back in a week or two, when perhaps I've recovered.  

Take me down to Kansas City

Where the rubric's clean and we take no pity

Oh won't you please take me home...