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19 April 2021

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!

13 April 2021

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.


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.

08 April 2021

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 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.

29 March 2021

Just the facts: Fluids for Physics 2

I'm doing a one-semester AP Physics 2 intensive course for some very dedicated students.  On a typical day we take a fundamentals quiz; we do a demo or three; then we spend lots of time doing problems and playing with the demo setups.*

*We did copious lab work in the first half of the year as part of our research projects.  And these students had a Physics 1 course in which we got hands on equipment during something like 75% of our classes.  I don't do so much formal lab work in this intensive, 'cause these folks have been there and done that; but they know how to play with equipment on their own.  At least when they're in person rather than online. :-)

I've had to construct an AP Physics 2 fact sheet.  Some of the facts can be found in other blog posts, like this one for magnetism or this one for capacitors or this one for optics.  

I've never published a fluids fact sheet.  Here's what I handed out today before I started lab work...


Static fluids

The pressure in a static column of fluid is P = P0 + rgh

            Here the rgh term is called the “gauge pressure,” meaning the pressure above atmospheric.

Density is defined as mass/volume.  Thus, mass can be expressed as rV.

The buoyant force on an object is equal to the weight of the fluid displaced.

The equation for the buoyant force is FB = rVg, where r is the density of the FLUID and V is the volume SUBMERGED.

Flowing fluids

The continuity principle is a statement of conservation of mass: the volume flow rate (or mass flow rate) must be the same everywhere.

The continuity principle for flow of cross sectional area A and speed v says A1v1 = A2v2.

Bernoulli’s equation is a statement of conservation of energy.

Bernoulli’s equation says P + rgh + ½rv2 is constant at any two locations.

25 March 2021

Atwood Machine: what if the rope is massive?

The question was asked on a physics teachers message board: How do you deal with a rope-and-pulley situation if the rope isn't truly massless?

I'm giving the AP Physics 1 answer here today.  This video by Pasco's Dan Burns gives a seriously intense mathematical solution.  You don't need that unless you're in and beyond AP Physics C, or unless you're solving the 2022 US Invitational Young Physicists Tournament problem on "rope and chain fountains."

I'm thinking of a modified atwood - one cart on a horizontal low-friction track, connected by a rope over a pulley to a hanging object.

Consider the entire cart-rope-hanging object system.  The unbalanced external force on the system is the weight of the hanging object.  (The force of the ropes on the objects is internal, because the rope is part of the system.)  Then set the system acceleration equal to this unbalanced force divided by the system mass.

But now give the rope a significant mass, still smaller than the mass of either object.  The unbalanced external force doesn't change (much)* - still the weight of the hanging object!  But with a larger system mass in the denominator, the acceleration becomes smaller.

*Okay, okay, the weight of the rope that's dangling contributes to this unbalanced external force.  So start the system with most of the massive rope horizontal.  And the rope's mass is still small compared to the hanging object's, so this is a good approximation.

Test this in class!  Use a smartcart on a track, connected by a thread over a pulley and onto, say, a 50 g hanging object.  Use the smartcart to measure acceleration.  Then remove the thread, and instead use a chain - like a bike chain, maybe? - to connect the smartcart to the hanger.  I'll bet you get a smaller acceleration!

This AP live video from April 2020 shows me doing this experiment with a low-mass rope, if you need ideas of how to make the acceleration measurement experimentally.  

21 March 2021

Teaching rotation as a cumulative mechanics review in AP Physics 1 or C

Where do you start with AP Physics 1 (or C) rotation, especially in a Pandemic Year when you might have a lot of virtual students?

Think of rotation as a vehicle for review-in-context of mechanics concepts.  Your class has studied the three main mechanics approaches:

  • Forces/kinematics, where we start with a free body diagram and/or a kinematics chart
  • Impulse/momentum, where we write the impulse-momentum theorem or p conservation
  • Energy, where we start with an annotated energy bar chart
The whole point of the rotation unit, to me, anyway, is to give students a new context in which they can review these three main approaches.  The AP Live videos in 2020 started with a focus on rotation, precisely because rotation is a great way to start a cumulative review.

So what do I do for each of these topics in rotation?

I personally don't do anything significant in class on rotational kinematics, since this is such a straightforward extension of linear kinematics.  I ask a few released AP multiple choice questions dealing with rotational kinematics graphs, and demonstrate the solution with the PASCO rotary motion probe. That's it.  

For newton's second law for rotation, I use the rotating platform and an in-class problem set.  By changing the lever arm for torque, the force for torque, or the rotational inertia, students predict how the angular acceleration changes.  This is essentially word-for-word what I did in the AP Live Newton's Second Law for rotation video, except as a come-and-show-me activity where they check their prediction in the back of the lab with the rotating platform.  This year, since we were 100% virtual in February, I just showed the linked video to the class over zoom.*

*Short rant: you've gotta watch the video with the class.  You can't just assign the video as homework.  How do I know?  I tried assigning some short - 8 minute! - AP Daily videos earlier this year via AP Classroom.  In class, it became apparent that very few had watched carefully.  Sure enough, the stats from AP Classroom showed that only about a third of my class had even opened the videos.  This in a class where I get better than 95% completion for daily problem sets!  The moral I drew was, don't ask students to watch videos on their own, 'cause that way madness lies.

For angular momentum and rotational kinetic energy, I use Pivot Interactives.  I've sometimes spent a full week on the "marble collides with block and can" activity, without much preamble beyond the fact sheet and 15 minutes showing demos about what rotational inertia and torque mean.  In the video, just asking "is linear momentum conserved?" gets students to think about center of mass motion and reviews linear momentum; then "is angular momentum conserved?" teaches them how to deal with angular momentum (especially for a point object) better than any lecture.  And finally, "is mechanical energy conserved?" forces them to draw an energy bar chart and include rotation.  I mean, truly, this is all I do to intro rotation.  We get straight into problem solving with rotational kinematics and N2L for rotation.  They don't need any presentation, because they already know the concepts and simply apply them.  

As you can see, most of this rotation introduction can be done virtually.  In my classroom after spring break - that's March 24 this year - I have students just doing independent lab work.  We do quizzes and short problem solving homework assignments that help not only review but also help teach rotation.  But that's it.  Between the AP live videos, Pivot Interactives, and some very open-ended lab work, my students get all the resources they need.  At this point, the class is full of experienced physics students.  They do well learning rotation sorta on their own, as I guide them to use rotation as a means to overall AP Physics 1 content and skill review.

07 March 2021

Not physics - question about English Pie.

In 2015, I was in London for a month on sabbatical.  I went to three football games, one Victorian pottery factory, toured several sports cathedrals and one school, and sat with the radio commentary team during a Manchester United-Aston Villa game.  As I looked through my notes recalling this amazing trip, I found a question about pies.  Perhaps an Englishperson could help me.

We went to Sainsbury's to shop for groceries for dinner on our second night in town.  My wife loaded up with all sorts of green things, but I found the counter with fresh pies.  I bought one of every variety of pork pie that was available.

Problem was, back at the flat, the oven started smoking when I turned it on.  (Don't worry, I turned it off.  No London Fire of 2015.)  I wanted a warm pie, not a refrigerated pie straight from the grocer's cold case.  

So, um... I used the microwave to heat each pie.

Was this a tremendous breach of Pie Etiquette?  Am I any less of a good person for having eaten microwaved pork pies?  

And when is Food Lion in Virginia going to start stocking these?  I'd probably keep them in business by myself.  

06 March 2021

Incentive other than a grade: exemptions.

Does every student need to do the same number of practice problems?

Early in the year, yes.  Even the best students need practice.  Even Allen Iverson needs practice.  And the top students gain confidence not just by doing their practice problems well, but also by helping their classmates. The not-top students gain confidence by occasionally being right when the top students are wrong.  

But as the year wanes?  Especially now, in Pandemic Times?  Awarding exemptions from practice problems can be a powerful incentive to students to take their work more seriously.

It's not the doing of the practice problems that develops physics skills - it's doing the practice problems carefully.  Even, especially, incorrect practice problems can help students learn physics.  But only if the student's incorrectness was born of a misconception, not of laziness or fatalism.  And only if the student cares about being correct.

When my students are all living together on dorm, when they can come to my classroom to work together the night before a problem is due, then it's easy to convince students to put forth serious effort on each problem.  The incentive isn't the grade - the incentive is, no one wants to see their name on the board.  The name on the board means to come in during an assigned free period to redo the problem.  It's not a punishment, but nevertheless... students work carefully the first time so that they're not likely to have to visit me for a second bite of the apple.

I can't use the name-on-board incentive method when we're online.  And since the students don't have in-person contact with any of their classmates, they don't work together; they don't incentivize each other through their presence.

So, I've taken to exempting students from the next assignment if they do well on today's assignment.  

Aren't these exempt students missing out on important practice?  Maybe.  Perhaps they are.  But is it really *important* practice for someone who's shown they can do the previous problem perfectly or near-perfectly?  I think a lot of teachers, parents, and students take it on faith that more practice is always better.  I disagree.  It's okay to let a student earn a break.

It's early March.  I already know which students care about grades and which don't, which students tend to get problems done for the sake of being done rather than for the sake of understanding the material.  These folks have already shown that the incentive of a job well done, the incentive of a high grade, isn't important to them.  So I need a different incentive.

I've seen students suddenly, for the first time all year, show tremendous effort, skill, and willingness to collaborate when they know that good performance will get them out of future work.  It's not a good idea to shame these folks with "you know, if you worked that carefully on every assignment, maybe you'd be doing better in this course."  They know.  Rubbing their noses in it is condescending and counterproductive.  I just congratulate such students on their exemption, and move along.

28 February 2021

Conceptual Physics Summer Institute 2021: July 24-25, or July 31-Aug 1

Folks, I'm already teaching several AP summer institutes - see the left-hand sidebar for details.  But what if you are looking for physics professional development that is NOT aimed at college-level physics?  I mean, I meet so many of you each year who teach on-level, honors, college-prep, Regents... to all ages, to all varieties of student.  And in my personal mission to spread physics knowledge to as wide an audience as possible, these sub-college courses represent a critical first point of contact with our discipline.  I focus as much energy on my conceptual course as on my AP course each year.  So I'd like to focus some of my summer professional development expertise on those who teach these first-level courses.

We did this last year, in 2020.  See the comments in this post for participant reviews.  They all say, this institute was an amazing, friendly, and productive experience.

I'm offering a two-day institute on July 24-25, or July 31-Aug 1.  These will be online, broadcasting via Zoom from my lab.  Skip past the institute description for fees and registration instructions.  Each weekend will be limited to the first 30 who sign up.  The daily agenda is included here at the bottom of the page.

Jacobs Physics
Conceptual Physics Institute Description
July 24-25 2021 or July 31-Aug 1 2021

All levels of high school physics can be taught conceptually – where verbal and experimental reasoning is prioritized over mathematical problem solving.  While mathematics are used extensively, they are used as a tool to create predictions about the workings of the natural world.  Whether you teach “general”, “on-level”, “honors”, “Regents”, or “college-prep” physics, a conceptual approach can be adapted to most any introductory physics topic – and to most any state or district standards. 

In our institute, we will discuss, practice, and share methods of teaching common physics content in a conceptual style.  I will be broadcasting from my laboratory via zoom.  Time will be devoted to experimental methods that are especially useful at the sub-college level; to course planning on a year-long and a unit basis; and to best-practices physics pedagogy, which differs substantially from pedagogy in other disciplines.

Participants will be given a full-year’s set of classroom-ready materials, including fact sheets, in-class and laboratory activities, assessments, and planning documents.  More importantly, through their interactions with the instructor and with their colleagues, participants will develop skills and ideas for adapting these materials to their specific classroom environment.  Those attending will also earn a certificate indicating their participation in 15 hours of physics professional development.

How much does it cost:  $200 for the weekend.  The schedule of events is listed below.

How do I register?
(1) Click the "donate" button below (or in the left column of the blog).  It will take you to paypal.
(2) Enter $200.00 as the donation amount, either through paypal or credit card
(3) Click "Add special instructions to seller" or "Add a note"
(4) In the note, please include your name, preferred contact email, and institution
(5) Fill in payment info and click "donate now"

That's all - I'll be back to you within a day or two confirming your registration, and sending you links to the classroom-ready materials.

Cancelation issues: If you register then can't attend, contact me via email.  As long as I can replace your spot, I'll send a full refund; if I can't replace your spot, I'll refund all but $25.

Schedule: Each session will include both whole-group presentation/discussion, and breakout groups for activities.  In between sessions and during breaks, Greg will be available for informal conversation. 

Saturday 24 July or 31 July (all times eastern time)
10:00                 Introductions
                        What does “conceptual” mean – defining levels of physics
Different levels of physics: developing your program
                        Different levels of physics: developing your resources

11:30               Eight styles of physics laboratory activities
            Including the two best-adapted for conceptual physics
My first day activity – reflection experiment
My first group laboratory experiment – refraction

1:00-1:30    break

1:30               Sequencing your course
Starting the year right: the most important physics teaching skill
Justifying answers with facts
Simple ray diagrams for optics in conceptual physics
Justifying answers with equations
In-class laboratory exercises: circuits

3:00                 The daily “quiz”
Tests and quizzes, targeted to different levels
                        Other sorts of assessment
                        Preparing for the trimester/semester exam
                        Adapting a conceptual course to external standards
                        In-class laboratory exercises: motion graphs

Afternoon – asynchronous
                        Read through the shared files
                        Read through the Jacobs Physics blog
                        Adapt to your district or state standards
                        Bring questions and ideas for the social or for Sunday

7:30                 Optional Zoom social: Dinner, dessert, drinks, and conversation.  BYOB, obviously. 

Sunday 25 July or 1 August 
10:00                 Building and creating experiments with whatever you’ve got
                        Developing your own in-class lab exercises
                        Using or substituting inexpensive equipment
                        In-class laboratory exercises: direction of force and motion

11:30               Methods to speed your grading
                        In-class laboratory exercises: forces in 2-d
                        In-class laboratory exercises: motion in 2-d
1:00-1:30    break

1:30               The final third of the year – once skills are built
                        How I teach impulse/momentum
                        Energy bar charts at the conceptual level      
                        Laboratory exercises with harmonic motion

3:00                 Sharing: Any Other Demos
Online simulations:
                                    The Physics Classroom
The Physics Aviary
                                    Vernier’s Pivot Interactives
                        Ending the year: the Physics Fight

26 February 2021

Dealing with g in conceptual physics - and a "false calculation" example

A correspondent has questions about how I deal with g in conceptual physics:

I notice that you intentionally use g as 10 N/kg instead of m/s/s and say that weight is the force of a planet.  I've noticed that when we get to energy, students struggle with g for GPE=mgh. They all want to put the mass (say 0.5 kg) but then they say 5N or 5N/kg for g.  It seems they have a very shallow understanding of what we're doing with 10 N/kg (and I notice you don't tell them explicitly to do mg for weight).  And the other thing they do is anytime they see a mass in kg, they'll "convert it to newtons." Or they'll see a force of 5 N and say it equals 0.5 kg.

Do you care whether they make the connection between "objects in free fall gain/lose 10m/s/s" and the gravitational field? For instance: when they're solving for speed based on KE, when N/kg doesn't fit with the other units. Or do you not really care about manipulating units, as long as they know what units go to what quantities (i.e. speed is always m/s, energy is always J).  

In conceptual, I don't care at all if they use careful language about g.  I never explicitly make the connection between the 10 m/s/s free fall acceleration and the 10 N/kg gravitational field.  They do need to put correct units on, of course... but I am fine with them "converting" 5 kg to 50 N.  As long as they sort of can get the correct values for weight and mass, I'm fine.  No unit manipulation ever.  

If they can do a calculation with mgh in a table, indicating correct units on m, g, and h, with J on the final answer, this is fantastic (and difficult for conceptual students).  At this point if they plug in numbers correctly in a table and get a unit wrong here and there, as long as the final units are right, I'm fine.  

Usually, I'm asking for a comparison between two situations, like "what happens to the potential energy of this 0.5 kg object when its height doubles?"  Here many students like to fall back on what they call a "false calculation" - plug in simple values to the formula for the first situation, then the second situation, and see how the results differ.  See below.  

Things to notice: 

* This student made the mass 1 kg to make the calculations easier.  I encourage that!  This is the conceptual physics version of "Bertha's Rule of Ones."  They chose 1 m and 2 m to double the height. If they had chosen 3 and 6 m, that's fine, too.

* I do expect units on their tables; I don't expect units on their arithmetic, because a lot of 14 year olds I teach have trouble multiplying (1 kg)*(10 N/kg), even though they can multiply (1)*(10) just fine.  Make the process simple, and likely to lead to the right answer.  

* Yeah, this student forgot units on g in the second table.  But they got 'em in the first.  I would likely let this go.  I mean, there's a line in the sand between doing math with no physics on one side, and pedantry on the other.  The goal is that by year's end, my students themselves should be uncomfortable if they left units off something.

What about in AP Physics?  I still start out not caring much about the true meaning of g... still no unit manipulation.  They do eventually need to understand that free fall g is the same as gravitational field g, because the AP exam discusses the difference between gravitational and inertial mass.  Yet, I don't even mention that difference or the connection until after spring break.  I'm far more concerned with their articulation of correct annotated energy bar charts; and with the conceptual difference between mass and weight.  (In AP, I do encourage students to write mg for the force of the earth on an object.)

You're right that student understanding of 10 N/kg is often quite shallow.  Usually they get it more as the year goes on; sometimes they don't truly "get it."  But if they can make proper energy bar carts and make correct qualitative and quantitative predictions with energy bar charts, then understanding the meaning of g can be left for a future class - or might never happen.