Fundamentals checks for AP Physics *2*

Last year I wrote a bunch of fundamentals checks in preparation for the AP physics 1 or C-mechanics exams.  This year, I'm teaching a physics 2 section.  So, I'm writing daily quizzes for P2 now!  I'll post the quizzes here, then follow up a couple of days later with the answers in the comments.

Quiz 1:

1.       A substance has index of refraction 1.5.  What is (or how do I figure out) the speed of light in this substance?

2.       Write the equation relating focal length, image distance, and object distance for a mirror.

3.       Write the equation for the energy carried by a photon.

4.       Under what conditions is the image distance d_i positive, and under what conditions is it negative?

5.       The diagram shows light hitting a mirror.  What is the angle of incidence as the light hits the mirror?

6.       A resistor is connected to a battery, and the current in the circuit is measured.  The voltage of the battery is increased, and the current is measured again.  Which Ohm’s Law variable(s) remain unchanged after the voltage is increased?

7.       Under what conditions is the electric field given by the equation E = kQ/d²?

8.       One mole of monotomic ideal gas begins at pressure P₁ and volume V_1­.  Next, the pressure is increased to P₂ without changing the volume.  Write an expression (including a + or – sign) for the work done on the gas.

 9.       Write the equation for the capacitance of a parallel plate capacitor.

 10.   A photon initially has wavelength 500 nm.  The photon collides with a free electron which was initially at rest, giving the electron 3.0 eV of kinetic energy.  The photon bounces back opposite the direction it was initially moving.  What is the speed of the reflected photon?

2024 Conceptual Physics Summer Institute - July 19-20, registration is OPEN!

This summer's session will be Friday-Saturday July 19-20.  We need a minimum of ten participants to run the institute.  In the unlikely event that we don't get ten, I'll refund all fees and you'll keep access to the shared files.  So sign up now, and I'll post here when we're confirmed!

If you're close enough to Woodberry Forest School, you're welcome to attend in person rather than online (though I'm prioritizing the online experience).  Two people chose this option last summer, and enjoyed it!

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've done this for four years now.  See the comments at the end of this flyer for participant reviews.  They all say, this institute was an amazing, friendly, and productive experience.

I'm offering a two-day institute on July 14-15, 2023.  It will be online, broadcasting via Zoom from my lab.  Skip past the institute description for fees and registration instructions.  The single session will be limited to the first 40 who sign up.  The daily agenda is included here at the bottom of the page.

Jacobs Physics

Conceptual Physics Institute Description
July 19-20, 2024

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?  Paypal, Venmo, or even a check!

For Paypal:
(1) Use this link, or the button in the left column of the blog.  It will take you to paypal.
(2) Enter $200 as the donation amount, either through paypal or credit card
(3) Click "Add special instructions to seller" or "click here to provide contact info"
(4) In the note, please include your name, preferred contact email, and institution
(5) Fill in payment info and click "donate now"

For Venmo, last four digits 3550, scan the code to the right here -->
Then email me greg underscore jacobs @ woodberry dot org with your name, contact email, and institution.

If you'd like to pay by check, just email me as above; I'll send an invoice that you can forward to your school for payment!

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.

We do need a minimum of 10 participants to run the session!  In the event that we don't get that minimum, I'll refund your registration fee; yet you will retain access to all the institute materials.   

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. 

Friday 19 July (all times eastern time)

10:00               Introductions

                        What does “conceptual” mean – defining levels of physics

Different levels of physics: Culture building in the physics classroom

                        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. 

Saturday 20 July

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

                         Ending the year: the Physics Fight

Energy of various systems in an inelastic collision

 

I was asked about the situation above, in which two carts of different masses are released from rest and roll down frictionless ramps.  The carts collide and stick together on the flat surface.

Let's treat this as a goal-less problem:

Edna, Bertha, and Anthony by @Aldescary

Which way do the carts move after they collide?  

Well, Anthony (whom Edna calls a mean hippopotamus) says that the carts don't move after collision - they both have the same energy but in opposite directions, which cancel.

Oy.  So many things wrong with Anthony's answer.  Let's start with the fact that energy is a scalar quantity - energy can't have direction at all, let alone "cancel" other forms of energy.  And whenever we see a collision, energy should never be the first port of call - momentum should be.

It is true that the gravitational energy of each earth-cart system is converted to kinetic energy at the bottom.  And since the gravitational energy 2mgd is the same for both earth-cart systems, each cart will have the same kinetic energy before collision.

However, the momentums of each cart will be different.  I like to use the shortcut equation K = p^2 / 2m in this case to see that with the same kinetic energy, the cart with a greater mass will also have greater momentum.  You could also convert gravitational energy to kinetic energy to show that the speed at the bottom will be the same for both carts with mass canceling; then by p = mv the bigger mass with the same speed has bigger momentum.

So the cart moving left has larger momentum than the cart moving right, meaning the two-cart system has a leftward momentum.  System momentum must be the same after collision as before, so the momentum is still leftward after collision... and that's the way the stuck-together carts will move.

Is mechanical energy of the both-carts-and-earth system conserved from release to just BEFORE the collision?

Anthony says mechanical energy is potential plus kinetic energy, and is always conserved, so yes.

Well, even a blind squirrel, or hippopotamus, finds a nut once in a while.  Anthony is pretty much correct.  Mechanical energy is conserved when no work is done by external forces and when no internal energy conversion occurs.  Here, the only external force acting on the carts-earth system is the normal forces of the surfaces on the carts.  These forces are perpendicular to the carts' motion, and so do no work.  Mechanical energy is, in fact, conserved here!

Is mechanical energy of the both-carts-and-earth system conserved from release to just AFTER the collision?

Anthony is perturbed... he already answered this question!  Mechanical energy is conserved, period, full stop, end of sentence.  Hemph.

Oh, Anthony... when carts collide and stick together, they undergo an inelastic collision by definition.  Mechanical energy may never be conserved in an inelastic collision - rather, some mechanical energy must be converted to internal energy.  

Is mechanical energy of the both-carts-only system conserved from release to just AFTER the collision?

Anthony says he's done answering these tricky questions involving systems.  He's gonna put his head down on his desk and listen to Edna for a change.

Edna thinks this one is pretty simple... because an object by itself can only possess kinetic energy!  (Potential energy can only exist when a spring or the earth is incl
uded in the relevant system.)  So the mechanical energy of the carts is just their kinetic energy.  On release from rest, the carts have no speed and therefore no kinetic energy.  After collision, the carts are moving, so they have kinetic energy.  The KE has gone from zero to not-zero, and therefore has increased.

Call for jurors: 2024 Conceptual Physics Tournament, on May 19. We pay $100.

In my school's conceptual physics program, we give cumulative written tests after the first and second trimesters.  In lieu of a final exam*, we are once again running the Woodberry Forest Conceptual Physics Tournament!  This is a competition for 9th graders, to be held at 1:00 on Sunday May 19 2024.  We've done this before, including the last two years after a pandemic-enforced break.  We're happy to be back to annual.

*No, to be clear to all, we're not giving an A to the winner and an F to the person in last place.  That's silly.  We're just having a fun, competitive tournament, to determine a winner.  Jurors engage in discussion and conversation with participants about their problems.  Jurors then award scores and write comments for the participants; jurors aren't assigning grades!

How does this tournament work?

On May 1, I will reveal a slate of three problems to the 64 participants.  These problems will be in the style of AP Physics 1 "paragraph response" questions.  Except, rather than just answer in a paragraph, the students will spend the month of May setting up experiments to provide evidence for their answers. By tournament time, each student will be expected to be prepared to discuss the solution to two of the three problems, with both theoretical and experimental support.

At the tournament, each student will participate in two "physics fights."  Think of these physics fights like a miniature version of a graduate thesis defense.  Students will have a strict limit of two minutes to present their solution to a group of two or three jurors, who then will engage each student in conversation about the problem for five minutes.  The students are evaluated by the jurors not only on the quality of their solution, but also on their ability to discuss the solution, to confidently hold a conversation with the jury.

Importantly, jurors are explicitly instructed on their primary role - to find out how much the students DO know, not merely to expose what they don't know.  

How do the students prepare?

Starting on May 1, all conceptual physics classes the rest of the year will be devoted to tournament preparation.  They'll work together to set up experiments in class, they'll be assigned to write up their solution as homework, they'll practice presenting.  They'll get intense instruction and guidance from the conceptual physics teachers, from their peers in the AP classes, from those who've been through this tournament before.

We need jurors.

The key, I think, to any class project is external assessment.  I and the other conceptual physics teachers will play the role of coach and advocate, always encouraging and helping the students to deepen their understanding of the problems and to improve their presentations.  Our relationship will be purely supportive, enthusiastic, positive.  

We can't then turn around and grill these same students as examiners!  That'd be like our football team's coaching staff refereeing the state finals.  Even -- especially -- if their officiating were fair, the coach-student relationship, both in before and after the game, would be irrevocably compromised.

So we need jurors.  We can pay.

Would you like to come to Woodberry on May 19 to be a juror?  We'd ask you to arrive at 11:30.  We'll have a meeting of all jurors in our beautiful dining hall over lunch.  

Then we'll ask you to be a juror for a couple of hours' worth of physics fights.  You'll be partnered with several other examiners over the course of the afternoon, getting to know a diverse set of fun folks from all over.  When all students have presented their two physics fights (to two separate juries), we'll gather the jurors for conversation, coffee, snacks, and their paycheck.

In any case, our goal is to be done by 3:30, or possibly 4:00 if there are logistical issues.  No later -- our students will be attending the final seated meal with their advisors that night followed by study hall, so we can't run late.

We will pay you $100 plus lunch for your time.  (If you're coming from more than a few hours away, we can put you up on campus on Saturday or Sunday night - please let me know if you're interested in this option!) I think you'd find that the camaraderie among the jurors and the engagement with the students will make the trip worthwhile.

Who's eligible as an juror?

Anyone who has passed a college-level physics class.  This includes alumni of your advanced physics class, even if they're still juniors or seniors in high school - we've had several teachers bring a caravan of students, and they've had an awesome time.  We've had local college or graduate students on juries, we've had parents, alumni, colleagues who teach other subjects, grandparents, friends... Anyone willing to engage in conversation about physics at the high school level, as long as you can recognize good and bad physics, we'd love to have you.  We are looking for a diverse juror pool, which especially includes diversity in age - truly, we want folks in their teens as well as folks in their 70s, and everywhere in between. When I ran the USIYPT, I found the mixture of undergraduate / graduate / professor / high school teacher / industrial physicist / retired physicist on the juror panel allowed some amazing relationships to develop.  I'd love to create a similar vibe here.

How can I sign up?

Send me an email via greg dot jacobs at woodberry dot org.  I'll send you more information, including the three problems, and our current draft of the scoring rubric.

We would like to get 45 jurors - pretty much the first 45 who sign up.  I can't wait to see some blog readers!  I'll even introduce you and your students to my pet hippopotamus, Edna.  :-)

April 8 2024: Free AP Physics 1 exam prep live show! (Archive link available)

McGraw-Hill, publisher of the 5 Steps to a 5 AP prep book series, is sponsoring a free live physics show for students and teachers.  I'll be presenting from my lab on Monday April 8, from 3:30-5:00pm eastern time.

All are welcome!  The way to join is, teachers (preferably) should "register" for free at this link.  They won't ask for you to make a username and password, nor to receive marketing emails.  Just give your last name and email.  Then, you'll get a message with the link to join - a link you can share with all your students, colleagues, friends, whoever.*

*McGraw-Hill would prefer not to be in the business of collecting student emails.  So they ask for a teacher's email, and ask the teacher to share the link.  They're trying to be entirely above board here!  There's no sneaky agenda to get your personal info!   The only agenda is, they want your students to buy the 5 Steps book.  :-)

I haven't planned the show yet, other than the general vibe, which will be similar to my 2020 live shows or my AP classroom videos (for Physics 1, units 2 and 3).  My pet hippopotamus Edna is excited for the event, and no doubt will make an appearance.

Do you have any requests?  My initial brainstorms are perhaps to set up experiments based on the 2023 P1 exam questions 1 and 2 - about harmonic motion, and a cart rolling down a ramp.  And I'll definitely leave plenty of time for improv, in which Bob the master of ceremonies will read the chat, and relay questions or requests to me.  However, I'm open to all sorts of suggestions now.  Post in the comments, or contact me via email or Bluesky!  I can do a lot in a 90 minute live show.  Tell me what you and/or your students want to see, and I'll try to make it happen!

And spread the word.  Last year, this Physics 1 live show was the best-attended of all of the subjects they offered... by a factor of about 20.  Let's keep that momentum going!  See you on April 8.

Update April 9 2024: The archive link is https://mcgrawhill.info/43yhAkc

AP Summer Institutes 2024 - will the new exam content and structure be discussed?

Of course it will!  

I'm doing several online and in-person P1 institutes in summer 2024 - see the sidebar to the left for details.  Please sign up!  The institute will certainly discuss content and structure changes for the 2025 exams.

What, specifically, will we do?

• We will do a number of fluids demonstrations and lab activities, including all three major fluids topics.
• We will do demonstrations with the three minor content changes in P1: parallel axis theorem, quantitative understanding of elliptical orbits, and center of mass location.
• We will discuss the new exam format - though I will emphasize that in P1, preparing students for the 2025 exam format looks exactly like preparing students for the 2015-2024 exam format.

Of course, the institute will still do all the AP Summer Institute things that you expect.  I'll give an overview of the AP program, the course audit, and AP classroom.  Much more interestingly and importantly, I'll discuss give you access to my course files, problem sets, laboratory activities, quizzes, tests, and a day-by-day planner.  Teachers new to AP Physics 1 can use these verbatim to get themselves started; veterans can use the materials to supplement and inform what they already do well.

The highlight of the in-person institutes is the "studio time" in lab on the final day, in which we all work together to set up and develop laboratory exercises based on released AP questions.  You'll come away with a dozen or more pre-tested new lab ideas!  In the online institutes, which are broadcast live from my actual classroom, we'll instead do "improv time" - challenge me to set up an experiment, or to show how I use demonstrations to teach any topic on the exam.

As of the 2025 AP exam revision, are Physics C mechanics and Physics C E&M two separate year long courses now? (No.)

On February 29 2024, the College Board released the course and exam descriptions for the revised version of all four AP Physics courses. You can find all the information and links at this page.  

For the AP Physics C exams, the course content will not change.  However, all AP physics exams - P1, P2, C-mechanics, and C-E&M - will be in an identical format as of the 2025 exams.  The format is, 80 minutes for 40 multiple choice questions; and 100 minutes for 4 free response questions.  That means an entire exam takes three hours.

But wait!  For decades, the two physics C courses have each had 90 minute exams, not three-hour exams, because the physics C courses have each represented a single-semester college course.  Has that changed?  

It has not.  Each of the FOUR courses now represents what is taught at the college level in a single semester.  AP Physics 1 represents the first semester of a college-level algebra-based introductory course.  AP C-E&M represents the second semester of a college-level calculus-based introductory course. And so on.

Thing is, we are teaching high school classes on a high school schedule.  The vast, vast majority of high school students taking physics for the first time should do a full year of mechanics.  This full year can be AP Physics 1; this can also be AP Physics C-mechanics for advanced students who are taking calculus.  Both cover substantially identical concepts.

For those taking a SECOND year of high level physics, well, AP Physics C-mechanics probably isn't challenging enough.  It's absolutely normal, acceptable, reasonable, typical for a student to take AP Physics 1 one year, then the combination of C-mechanics and C-E&M in their second year.  This post gives a recommended course sequence for such students.  

Me, I like to teach AP Physics 2 as my second-year high school course.  It's rich in diverse content, meaning that physics veterans won't ever say "oh, geez, not another cart on a ramp".  It's also particularly well adapted to seniors who need the course front-loaded - start with the hardest stuff like electricity and magnetism, and end with the simpler and more concrete topics like optics and thermodynamics.

But in any case, in any way you adapt the courses to your particular school ecosystem, the three-hour AP Physics C exams don't mean anything about how long you spend teaching the material.  Rather, the longer exams are a response to the fact that the pre-2025 APC exams were quite "speeded."  They were as much a test of how fast a student could do physics as how well a student could do physics.  And that's not what anyone wants to test.

Experimental procedures in AP physics, the redesigned free response section, and Wally the Astronaut

Wally the Astronaut, from The Physics Aviary

Above is a screenshot from the "Work to KE" simulated laboratory exercise on The Physics Aviary. In the exercise, you press start, and a fire extinguisher causes Wally the Astronaut to speed up.  You press stop, and the fire extinguisher ceases to apply a force.  Wally coasts, then passes through two photogates separated by 10 meters.  The time for Wally to cross the photogates is displayed.

You can do a thousand different sorts of classroom exercises with this single simulation.  I like to give this quiz here, go over the quiz, then have students go through this laboratory exercise. But the simulation here is so, so rich, you could do many different things.  Propose your favorite in the comments!

I was asked how I would describe* an experimental procedure on the AP physics 1 exam.  Some teachers ask their students to write a step-by-step instruction manual, including safety procedures and calculation instructions, for an in-class laboratory exercise.  Is that what the AP exam demands?  Should a procedure include calculational instructions?

*The "task verbs" on the AP physics exams will be in boldface as of the 2025 administration.

Historically, the AP readers expect students to communicate what they measured, and how they measured it.  If the experiment could in fact be done in a reasonable high school laboratory, the procedure is legit.  

The prompt on the AP exam - especially the redesigned 2025 AP exams - will be more targeted than what I often see in classroom lab handouts.  For example, the exam might write:

(a) Students are asked to take measurements to create a graph that could be used to determine the mass of Wally.  Describe an experimental procedure that the students could use to collect the data needed to determine Wally's mass. Include any steps necessary to reduce experimental uncertainty.

My response might be, "Measure the force F exerted by the fire extinguisher with a scale.  Then in each of many trials, turn off the fire extinguisher after Wally has traveled a distance d, a different distance in each trial.  Measure d; and divide the 10 m photogate distance by the time output of the photogate to find Wally's speed v."

The analysis - that is, how to do the necessary calculations - is usually in a separate lettered part of the question.  It's fine generally to write the analysis part in the same section as the procedure!  But the procedure will earn points independent of the analysis.  One being wrong or incomplete doesn't affect how the other will be scored.

Part (b) might ask about the analysis:

(b) Describe how the data collected in part (a) could be plotted to create a linear graph and how that graph would be analyzed to determine Wally's mass m.

And I'd say, "Wally's kinetic energy is equal to the work done by the fire extinguisher, .  [That first sentence is probably not necessary for credit!  But I write it so it's clear where my analysis comes from.]  Plot the work done by the fire extinguisher (Fd) on the vertical axis; plot (1/2)v^2 on the horizontal axis.  The slope will be Wally's mass."

Full credit would be earned for a more bare-bones "Plot Fd on the vertical axis, and v^2 on the horizontal.  The slope is (1/2)m."

A daily quiz based on 2023 AP Physics 1 question 1 - Did you *understand* how to do the homework problem?

It's getting toward the back half of the school year in AP Physics 1.  I've made a first pass at all the major content units; we've done laboratory activities out the wazoo.  We're gearing toward one more half-length practice AP exam before spring break, and then a final half-length practice in mid April.

My students need practice doing cumulative, AP-like problems which require synthesis of multiple concepts; or which require students to choose from the entire year's menu of possible approaches.  Later on, in April and May, I'll have students do authentic AP free response questions in class practically every day, without a safety net.  We're not quite ready to take the safety net away.

No, right now, I'm assigning AP-style free response questions as collaborative out-of-class work.  Everyone is encouraged to collaborate, to seek help when they're stuck.  As long as they get to the correct answers eventually, I'm happy that they're making progress.

You have questions about this approach.  "Even the most honest, diligent students will often just do what their smart friend told them to do, Greg.  Getting done with the assignment is more important than getting it done right.  Even with the five-foot rule religiously followed, at least some students are parroting, not learning, not progressing."  

Unless there's disincentive for pure parroting.  And I don't mean grade disincentive.

The approach I use - which is absolutely not the only effective approach! - this time of year is the daily quiz based on the AP-style problem.  When students come to class, I collect their assignment.  But the first four minutes of class are basic questions about the problem they did for homework.  We trade and grade the quiz, then I collect the quiz.  

Someone who understood the problem, even if they had to be nudged hard in the right direction, can do the quiz just fine.  Someone who truly parroted the smart kid cannot do the quiz.

Yet!  Even the student who parroted and then flunked the quiz has made progress!  The point of the quiz isn't to play gotcha, it's to review the problem in a context in which the students will listen.  If I say "Imma go over last night's homework," no one cares.  But if I say, "here's the answer to question 1 on the quiz and how I know, now mark your classmate's paper right or wrong," I get rapt attention.

My class is contract graded, which means there's no shame for poor performance, no cookie for being perfect.  What's the incentive, then, to take the assignment and quiz seriously?  If someone does particularly poorly on the quiz or problem set, I bring them in for a consultation to redo the quiz.  I just had a student in while I was writing this post.  It took him a relatively short time to redo the problem perfectly, with clear justifications for each part (including the parts that didn't initially require justification).  He didn't get this problem at first, but the combination of attempting it for homework, trying the quiz, and grading someone else's quiz meant that he gained a serious understanding of this problem.

Your ideas are intriguing to me, and I wish to subscribe to your newsletter.  Okay, here's issue 1: a quiz based on the 2023 AP Physics 1 exam problem 1.   Notice how the quiz gets to the essence of the solution without just asking "what was the answer".  This quiz brought forth excellent questions from the class about the physics behind the original question.  It made them think!

A cart oscillates, as shown above and on the problem set last night.

1. Point A on the graph is labeled in red.  On figure 1, draw and label where the cart is located at position A.

2. Point B on the graph is labeled in blue.  On figure 1, draw and label where the cart is located at position B.

3. How is frequency related to period?

4. What is the equation for the period of an object on a spring?

5. When a block is dropped on the cart, does the frequency of oscillation increase, decrease, or stay the same?

6. When a block is dropped on the cart, does the amplitude of oscillation increase, decrease, or stay the same?

7. When a block is dropped on the cart, does the maximum potential energy of the cart-block-spring system increase, decrease, or stay the same?

8. When a block is dropped on the cart, does the maximum kinetic energy of the cart-block-spring system increase, decrease, or stay the same?

9. When a block is dropped on the cart, does the maximum speed of the cart-block-spring system increase, decrease, or stay the same?

Mail Time: how do I have students describe normal and friction forces?

Vanessa asks:

How do you have students list the normal force and friction force on an object experiencing friction? Would both Fn and Ff be described as "the force of the surface on the object"?

Or do you have them specify "the normal force of the surface acting on the object" and "the friction force of the surface acting on the object"?

Just "force of track on cart" or "force of the ground on the cart" or similar, like you said.

I work so hard to get students to avoid excess language (like "the downward force of the earth pulling down on the upward moving cart") that I'd undo that work if I insisted on other language.  The simplicity helps substantially with Newton's 3rd law, for which we just switch the objects experiencing and applying the force.  

The 3rd law force pair to the friction force?  Well, friction is the force of the track on the cart, so the 3rd law pair is the force of the cart on the track.  That easy - but only if the friction force is originally written with this concise language.