25 March 2015

Need a paragraph-response item for AP Physics 1?

Remember the 2013 AP Physics B problem 4 about a modified-Atwood machine?  A ball was placed on top of the block on the top surface.  That ball became a projectile when the top block hit the edge of the table.  After some good calculational parts, part (d) asked a very carefully targeted descriptive question.  The exam stated that increasing the mass of the ball would cause the ball to land closer to the table... then asked, why?

I'd say a good quarter of the responses globally were something along the line of "since the ball is heavier, it falls faster, and so doesn't have time to go as far while it's in the air."  My personal opinion -- NOT shared by the College Board, at least publicly -- is that readers should be allowed to make a student who gives this response retake the entire test, this time in the presence of flesh-eating ants.  My proposal for this particular adjustment to the rubric was not accepted by the table leaders, because it was too late in the process to make substantive changes.

Every physics teacher I know praised the verbal portion of the question.  It did an outstanding job of rewarding students who could *explain* physics rather than merely solve problems.  On the old AP Physics B exam, a verbal-response item had to be targeted carefully such that students could answer in the time allotted.  The new exam provides plenty of time for such questions.  In fact, one entire question will include a phrase such as "justify your answer in a clear, coherent, paragraph-length response."  I rewrote this 2013 AP B problem as a paragraph-response item.  Here is my version of the problem, followed by some commentary.




1.      (7 points, suggested time about 13 minutes)
A ball of mass m is in a cup of negligible mass attached to a block of mass M that is on a table.  A string passing over a light pulley connects the blocks to a 2.5 kg object, as shown above.  The cup is a vertical distance h off of the floor.  All friction is negligible.
 In Trial 1, the system is released from rest, the block accelerates to the right, and after moving a distance x the block collides with a bumper near the end of the table.  The ball continues to move and lands on the floor at a position a distance d horizontally from where it leaves the cup.  In Trial 2, the mass m of the ball in the cup is doubled.  The system is again released from rest.  The block collides with the same bumper, the ball continues to move, and lands on the floor.
 In trial 2, does the heavier ball land a horizontal distance from the table that is greater than, less than, or equal to d?  Justify your answer in a clear, coherent, paragraph-length explanation.

Since the paragraph-response item doesn't ask for calculations, I eliminated most of the numerical values.  (I left the 2.5 kg hanging mass so I didn't have to make up yet another mass variable.)  In thirteen minutes, a student will have plenty of time for reasoning and writing; so I didn't feel the need to reveal that the ball goes a distance less than d.  

We only have one published example of a paragraph-response rubric, so most of us teachers are on our own to guess how these will be scored.  In my rough rubric, I looked for the following seven elements:

* Recognition that it is necessary to consider the ball's speed after traveling the distance x
* Statement that the ball has a smaller speed after the distance x in trial 2
* Then two steps of reasoning supporting a smaller speed, for example:
       + In F=ma, the net force on the system is the same with larger mass, giving smaller a
       + Smaller acceleration means the block speeds up less in trial 2, or reasoning with v2 = 2ax
* Statement that the ball goes a distance smaller than d
* Then two steps of reasoning supporting a smaller distance, for example:
       + Kinematic justification that the ball is in the air for the same time either way
       + Linking the horizontal distance to the initial horizontal velocity through x = vt

You might come up with a completely different rubric.  That's fine.  I merely chose several elements of reasoning that I expected to see, which allowed me to award partial credit to those who had a reasonable but incomplete understanding of the problem.

When I gave this item on my trimester exam, I found my seniors knocked it out of the park.  The continual writing practice we've been doing this year paid off big time.  The majority of the class earned full credit.  A few people said the ball would go the same distance either way, but they earned partial credit by correctly explaining the projectile portion of the problem -- they didn't understand the modified Atwood part of the problem.  My 9th graders didn't do as well, because they have a lot more trouble expressing their understanding in words.  They would have performed equivalently or better to the seniors on the original, calculation-heavy 2013 AP Physics B problem.  But the seniors have so much more facility with the written word that they dominated the freshmen on this one.  Of course, that's fodder for a future post.

Please feel free to use this question in your class.  Let me know if you have tweaks, either for the question or for the rubric.  And feel free to send your paragraph-response items!

GCJ


16 March 2015

Two Confusing AP Physics 1 Learning Objectives: Waves

A friend asked about two of the new AP Physics 1 learning objectives.  He's (rightfully) confused about how to present them to his class.  They are:

6.A.4.1: The student is able to explain and/or predict qualitatively how the energy carried by a sound wave relates to the amplitude of the wave, and/or apply this concept to a real-world example.

and 

6.D.4.1: The student is able to challenge with evidence the claim that the wavelengths of standing waves are determined by the frequency of the source regardless of the size of the region. 

I'd say, it's not worth looking this closely at the curriculum framework.  A College Board speaker last year at the AP reading emphasized that straight-up teaching to the learning objectives could lead to disappointment.  She said that a number of teachers to the redesigned biology course had hammered their students with practice tasks and questions narrowly tailored to each learning objective, only to find that the students had trouble handling the broader free response items.

But you have the right general idea about using the curriculum framework to figure out what aspects of waves you need to present to your class.  Try looking not at the learning objectives, but instead at the "essential knowledge" statements.  They state the aspects of each topic that must be covered.  The learning objectives are hyper-specific about skills and "science practices."  Learning objectives are, in my mind, only useful to those asked to write test items; the true outline of the course comes from the essential knowledge statements.  

For the specific topics in your inquiry:

Essential knowledge 6.A.4 merely states the fact that energy carried by a mechanical wave depends on amplitude; and reminds us that examples should include sound waves.  (Not light waves -- that's in AP Physics 2.  We're talking waves on a string, waves in water, sound waves, etc.)  That's easy enough to teach, demonstrate, and explain.  I based a multiple choice question in my book around a stone thrown into a calm pond: the stone creates a wave on the waver's surface.  That wave would carry more energy per meter to a close-by shoreline than a far away shoreline.  And, sure enough, the amplitude of this wave would be higher near the close-by shore than the far away shore.

Essential knowledge 6.D.4 says "The possible wavelengths of a standing wave are determined by the size of the region to which they are confined," which is a highfalutin' way of describing the classic pictures of standing waves.  In language tied to the demonstrations my students have seen, I'd say that the length of a string or pipe must be just right to fit a whole number of standing wave "humps."  This essential knowledge statement goes on to remind us that changing the boundary conditions or the length of the string will change the possible wavelengths.

To me, this just means "teach standing waves."  Now, some folks think of "teaching standing waves" as simply memorizing the equations for fundamental and harmonic frequencies and wavelengths.  No, teaching about standing waves means showing demonstrations and experiments,  It means explaining why standing waves do or do not form.  It means being able to explain with diagrams and words how standing waves in an open pipe differ from those in a pipe closed at one end.  It means relating the relevant equations to those diagrams and words.  

When your students can answer any question about standing waves, including descriptive and experimental questions, you have taught them the necessary background for not just LO 6.D.4.1, but for the AP Physics 1 exam in its entirety.

GCJ
 

09 March 2015

Video Documentary: The US Invitational Young Physicists Tournament

Woodberry Forest's second place 2015 Physics Team
Woodberry Forest School hosted the US Invitational Young Physicists Tournament in January 2015. The school hired Kirby Martin to make a documentary video.  The documentary focuses on Woodberry's team and our research program; it also discusses the structure and benefits of the USIYPT.

I encourage you to take a look here at the documentary.  If you'd like to become involved in the tournament, send me an email; you can take a look at the problems for the 2016 tournament here.  

And in the world of crazy youtube algorithms:  The first follow-up video that youtube played for me after the USIYPT documentary was four hours of footage from day 3 of an NCAA Bowling invitational tournament.  

GCJ

05 March 2015

Does a "paragraph response" in AP Physics 1 require sentences? Or are bullet points enough?

One of my friends who's a bloody awesome College Board physics consultant has a follow-up question to the "white paper" about paragraph response free-response items on the new AP 1 and 2 exams.  She says she's had several teachers ask "hard and fast" if the CB need the answers in complete sentences, paragraph style; or if a numbered list or bullet points would suffice.

Thing is, I don't know for sure, because grading an AP Physics paragraph response item is new to pretty much everyone in the world.  I can make a well-educated guess, based on the scoring guidelines on the published practice exam.  Nothing in these guidelines says anything about complete sentences; it talks about a "coherent argument".  My instinct is that if I got bullet points that addressed the correct issues and formed a logically-connected, coherent argument, I'd be fine with that, even if some of the entries weren't complete sentences.  

That said, bullet points that DON'T make an obvious coherent "argument" won't work.  Saying "*potential energy mgh, kinetic energy 1/2mv^2 -- both blocks, momentum conserved, energy shared, move both ways" doesn't cut it.  "Oh, but I addressed each of the points in the rubric," says the student.  No, you didn't -- you didn't COMMUNICATE.  The response must make sense on first reading; the reader is not going to make connections for the student, the student must make connections for the reader.

What I'm sure many teachers are concerned about is the silly fifth grade social studies "answer in a complete sentence" meme.  Remember, the end-of-chapter question asked, "What are three major exports from the state of Texas?"  You wrote, "oil, beef, and football."  But your teacher condescendingly marked the answer wrong, saying "You didn't answer in a complete sentence."  Really?  You directly and accurately answered the question that was asked.  Turns out she expected you to restate the prompt in the answer, saying "Three major exports from Texas are oil, beef, and football."  The smart students who eventually became physics teachers considered that this teacher was making idiotic, anti-intellectual, bureaucratic demands that had no relation to the content being developed.

Had the book instead asked, "Based on the reading, describe several features of the Texas economy," then my initial "oil, beef, and football" response could justifiably be ignored.  In that case, *I'm* the idiot for giving a four-word answer to a complex question.  And if the question prompt included direction to answer in a clear, coherent paragraph, then I'm charged with an additional count of "failure to follow directions" on top of impertinence, laziness, and general wrongness.

In a physics context, the essay question will not ask "How fast will the ball go when it hits the ground?"  The test will ask something much more deep, which cannot be justified in one word or one equation.  Communication of complicated concepts will be required, and such complex communication generally requires sentences with subjects and verbs.  No one will be grading your students' grammar.  If they structure their response with bullets, I suspect that's fine... but a good response will *of necessity* include many sentences in those bullets, and those sentences will be logically connected.

Hope that helps.  I'll know more once I'm at the reading.  I'm a table leader for Physics 1 or 2... I'd actually love to try the essay question, even though it will make my brain hurt.  :-)

04 March 2015

Open-ended lab exercise: is it constant acceleration?


The College Board has published a set of "Inquiry-Based Laboratory Investigations" for AP Physics 1 and 2.  The downside is that, in order to get to the actual investigations, you have to sort through page upon page of ed-school baloney about "learning objectives" and "building a community of learners" and "exemplifying technology-enhanced interfaces in data-driven schools."*  


* I might have made that last one up with the educational jargon generator.  But check the CB publication to be sure.

The UPside -- and it's a pretty important upside -- is that the investigations themselves include lots of excellent lab ideas from really good physics teachers.  Don't think of these "investigations" as lab guides which you must use verbatim.  Use them instead as a teacher-tested resource to give you good ideas, especially for more open-ended experiments in which students have to design their own procedures.

Now, I'm a big fan of such open-ended exercises, but I only use them in the latter half of the school year.  My students don't come to me with enough basic lab skills to dive straight into even "What factors do and don't affect the period of a pendulum?"  I spend the first part of the year riding herd about collecting lots of data across an entire parameter space, linearizing a graph, using the slope and intercept of a best-fit line to determine physically meaningful quantities, etc.  Once my students begin to see the laboratory process as a bit repetitive, then it's time to give them open-ended challenges.  A couple weeks ago I did "Does a rubber band obey Hooke's Law?"  I submitted this elaborate writeup for the College Board; but in class, I present just the question, and let the students take it from there.  By the time we do this experiment, the principle that they must make a graph and use its best-fit line is well ingrained such that it doesn't even need a discussion.

The very first investigation in the CB's Laboratory Investigations publication put a marble on a track, and asked the student to design and carry out a procedure to determine whether the marble's acceleration was or was not constant.  The experimental setup precluded simply using a motion detector to check for a linear velocity-time graph -- a standard motion detector can't read the marble.  So students have to use stopwatches and metersticks, or video analysis.  In either case, it's a non-trivial exercise requiring significant physics comprehension to explain how to translate from the raw data -- which only show position and time -- to instantaneous acceleration at several locations along the track.

I may come back to this particular exercise in my laboratory later this year.  But for now, I adapted the question as a Direct Measurement Video homework assignment.  This video shows a wind-up toy car speeding up across a table.  I simply ask, "How would you determine whether the toy bus's acceleration is constant? Answer in a clear, coherent, paragraph-length response."  

 I was glad I waited until last week to give this assignment, even though we covered kinematics long ago.  The class had a lively discussion the next day -- a discussion that wouldn't have happened so readily earlier in the year while the class was more answer-focused, while the class was less confident in the difference between acceleration and velocity.  Even those who got the problem wrong on the homework understood their mistakes.  We discussed how to estimate instantaneous velocities at different positions.  We discussed whose method of determining instantaneous velocity was best.  (Early in the year, the question would have been "but is mine right or not?"  Now we have enough experience to understand varying degrees of accuracy in an experimental situation.)  

I'll save the numerous methods of determining whether acceleration was constant for a future post -- feel free to share your idea in the comments.  Or, better yet, give this assignment to your class.  See how they do.  Whiteboard the student-generated results.*  Tell me and other readers your ideas for follow-up questions.  In other words, use the comments to talk shop.

* Sorry, jargon generator again.


25 February 2015

Two new labs for AP Physics 1 waves

I was asked via email how I've dealt with waves in AP Physics 1.  Remember, my approach will change over the years, as I see the sorts of things asked on the exam, and as I get new ideas from shop talk.  For now, I've started by teaching AP Physics B waves, with a bit more detail about how standing waves are formed, and no credit for just mimicking the equations fn=nv/2L etc.  In order to develop a deeper understanding of standing waves, I tried two new labs this year:

(1) I used the pasco wave generator at the constant 60 Hz frequency attached to a long string.  The string was attached over a pulley, with a hanging mass providing a tension.  I had students change the tension (which changes the wave speed), and measure the wavelength of the resulting standing wave from node-to-node-to-node.  

So we didn't spend years pounding the calculator, I provided a lookup table mapping each hanging mass to the correct wave speed.  I used excel ahead of time with the linear density of the string that I measured and the equation v = root ((tension) / (linear density)).  This graph is linear; the slope was 60 Hz, which was the frequency of the generator.  Each group matched the 60 Hz frequency within their determined uncertainty.

The real pedagogical purpose of this experiment was to give students kinesthetic experience with standing waves.  I did not introduce harmonics to the class before this experiment!  We only discussed how standing waves are the result of interference between periodic waves traveling in opposite directions in a fixed space; and I showed them that the wavelength was twice the size of one "hump."  They found out for themselves that sometimes these standing waves didn't form -- they had to move the generator left and right to adjust the string length in order to get the standing waves to show up.  That was a nice transition into harmonics, and to the next experiment.

(2) I used adjustable-length pipes, open at both ends, with an iphone frequency generator to produce resonance in the tubes.  Each group plotted their pipe length at resonance vs. the frequency of the generator, changing the freqency in small increments so as to remain at the same harmonic.  Each group was then asked to make a linear plot from which the harmonic number that they used could be determined.  

Since we did this experiment after the one described above, it reinforced the condition under which standing waves occur.  When on subsequent homework a student was confused about standing waves questions, I explained in terms of these two experiments -- the pipe didn't resonate except at one or two special lengths, just like the string didn't show the humps unless you got the string length just right.  And just as you could lengthen the string by exactly one hump and get standing waves again without changing the frequency, you could lengthen the pipe for the same frequency and get another resonance.  How far would you need to lengthen the pipe?  One "hump" in the standing wave, i.e. 1/2 wavelength.

Now, there's more to be done, of course, but this is where I started.  In Physics B I might have done the first of these labs; I have more time in the new course, so I added the second.

As for homework or test questions to ask... check out the experimental question from the 2012 Physics B exam.  It proposes a similar experiment to number 2 above, but asks for a determination of the speed of sound.  That's a good follow-up a couple of weeks after the waves unit.

16 February 2015

White Paper from the AP Development Committees: "Paragraph Response" Expectations

The College Board has published several paragraphs detailing what the readers will be looking for in answers to "paragraph response" questions.  Take a look here.  I don't have much to add to their statements, which I think are clear and useful.  I will likely pass out their page-long discussion for my students to read during our exam review.  

The two points I'd highlight:

(1) "It should make sense on first reading."  Your students don't get to come to Kansas City in order to follow their exam from reader to reader saying "let me explain what I meant."  You only get one shot -- do not miss your chance to blow the reader away with your logical arguments.

And thus, in your class, don't allow a student to argue about his score on this kind of problem.  If it didn't make sense to you ON FIRST READING, it's wrong -- and you have backup on that point from the AP Physics Development Committee itself.

(2) "Full credit may not be earned if a paragraph-length response contains...: 
* Principles not presented in logical order
* lengthy digressions within an argument
* primarily equations or diagrams with little linking prose."

In other words, it's a paragraph -- use your words, younglings, and stay focused.  You can not earn credit by throwing everything that comes to mind at the wall and hoping something sticks.

The College Board has released one paragraph-response question each for AP Physics 1 and 2, in the free response section of the practice exam.  I have one more about static equilibrium that I wrote for my tests that I'd be happy for someone to post on PGP-secure -- please, someone email me, I'll send you the file, and you can post it for me.  Anyone else have good, vetted paragraph response problems?  

GCJ

14 February 2015

First exercises in rotation: Newton's Second Law and Rotational Inertia

At this point in AP Physics 1 for upperclassmen, students are used to the idea that a new topic brings new facts and equations, which are then applied to make predictions to be verified experimentally.  Rotation provides an opportunity to introduce the topic directly with individual laboratory exercises, especially since the concepts of inertia, force, and acceleration are already familiar.  We are simply applying the concepts to a rotational setting.  

I spent way too long creating three versions of the pictured setup.  An object of mass between 10-200 g hangs from a string, which is passed over a pulley and wrapped around an axle.  The axle is attached to a wide and massive disk.  The hanging object is released from rest, causing the disk to accelerate rotationally.  I have the expensive PASCO version; a similar apparatus can be created for just a few dollars from PVC pipe placed over a ringstand, kind of as shown on problem 3 of the 2001 AP Physics C Mechanics exam.

A set of seven exercises is available for you to download and try out at this link.  I hand out the first to everyone, and help each student create an angular velocity vs. time graph for the rotating disk.  Then, each student individually, with his own unique graph, answers each of the questions, getting my approval before moving on to the next one.

Creating that ω vs. t graph requires some ingenuity.  The simplest way is to use the smart pulley and photogate; in fact, the PASCO apparatus provides a screw specifically aligned so that the photogate can easily be placed just right.  Problem is, I have Vernier photogates, which conveniently don't fit with the PASCO equipment.  D'oh.

So I set up a photogate vertically, a bit more than one disk-radius away from the rotational axis.  I cut a piece of paper such that its width was a known angle -- I calculated that with the 11.7 cm-radius disk, a 0.9 cm width paper subtends an angle of 0.08 radians.  Don't ask me why I chose that value -- I did, it works, and now I ain't gonna do any more cutting and taping.  (You can check my math, though.   Using x = rθ, 0.9 cm does in fact equal 11.7 cm times 0.08 radians.)  

Next, I set up my labquest to read the photogate in "gate" mode, with a "distance" of 0.08 m.  Thus, the labquest thinks it's making a linear velocity vs. time graph in units of m/s.  I fooled it, though -- it's really making an angular velocity vs. time graph, in units of radians per second.


Once I finished the setup, the data collection was a breeze.  Students on their own could create beautifully linear ω vs. t graphs, as shown to the right.  Then they figured out to take the slope to determine the angular acceleration; they calculated torque with force times lever arm; and they calculated the rotational inertia of the disk.  I deliberately had half the class using the gray disk by itself, and the other half using the gray disk with a heavy ring on top; sure enough, the half of the class with the extra heavy ring calculated a significantly larger rotational inertia.


The subsequent exercises each ask the student to redo the experiment, changing one of three things:

(1) Changing the net torque by changing the hanging object's weight
(2) Changing the net torque by changing the lever arm
(3) Changing the rotational inertia by adding or removing the heavy ring

In each case, students predict the new angular acceleration using semi-quantitative reasoning, and then measure to verify their prediction.  Results are generally accurate well within 10% of the predictions.

Postcript:  Throughout these exercises, I'm making the approximation that the tension in the rope that provides the torque is equal to the weight of the hanging object.  This is not precise -- since the hanging object is accelerating downward, the tension in the rope is a big less than the weight of the hanging object.  So what.  When I do the precise calculation, I find that the effective rotational inertia of the whole system is increased by mr2, where is the hanging object's mass and r is the lever arm of 1-2 cm.  Since that is SO much less than the rotational inertia of the disk itself (where the disk's mass is significantly larger than m and the disk's radius is ten times larger than r), I've made a good assumption.  Eventually, when someone asks about the string's tension not being truly equal to mg (as someone did on Thursday), I can have a nice conversation.


05 February 2015

Use a quiz question to set up a lab investigation

I give daily quizzes with a variety of purposes.  And my lab exercises in AP Physics often involve creating a curved graph with direct data collection, followed by linearizing that graph.

Historically, I've struggled getting students to understand graph linearization.  Only a few students have truly understood how to figure out which variables go on which axes, and what the slope of the graph means.  Most of the class has needed multiple consultations with me and with their friends to get each experiment done; and more often than not they haven't been able to reproduce their analysis later on.  Graph linearization is abstract and difficult.

This year, I made graph linearization a common topic for daily quizzes.  I started simply:  

 I make a graph of the net force experienced by an object on the vertical axis, and the acceleration experienced by the object on the horizontal axis.  What is the physical meaning of the slope of the graph?
I teach that we solve the relevant equation for the vertical axis... then using the equation y=mx+b, identify the y and x variables.  What's left is represented by the slope of the line.  In this case, the relevant equation is F=ma.  The vertical axis is F, the horizontal axis is a, so the slope is the cart's mass.

The biggest misconception is to deal with units not with variables in an equation.  Someone will get the answer right by saying "the units are N/(N/kg), which is kilograms.  That's mass."  Well, that sometimes works.  It sometimes is too difficult to mess with (i.e. for those who don't recognize alternate forms of units for N or m/s/s).  And it is very often wrong.

In the lab, a student releases cars from rest on an incline, and measures the distance they travel on the incline.  The relevant equation is x = vot + ½at2, with vo = 0. The student keeps the time of travel constant while changing the cart’s acceleration.  He graphs the distance traveled on the vertical axis, and the acceleration on the horizontal.  What is the meaning of this graph’s slope?

Now the vertical axis is distance x, and the horizontal axis is a.  That leaves the slope as (1/2)t2.  The student doing a unit analysis might get the t2 part, but he certainly won't get the factor of 1/2.

Things get even more complicated when I ask students to figure out for themselves what to graph.  But I'm still using daily quizzes to get them to practice -- primarily because I can do one every day or two, and give them instant and brief feedback on their answers.  

Last week I did the standard period-vs.-mass-of-a-spring experiment.  I have students collect period vs. mass data, then they linearize such that the slope of their graph allows determination of the spring constant.  When they're all done, I use my five-second spring constant measurement method to check each group's result.  

Try this quiz.  It asks directly what a graph of period vs. mass for a spring looks like.  (Learning to sketch the shape of a graph is a different skill that I'm also working on through daily quizzes.)  Next, it asks for a possible linearization and the meaning of the slope.  

Not only do we go over and grade this quiz for immediate feedback, we go straight into the lab to do the experiment.  I've primed my students' brains to know what to expect from the experiment.  Then when it's time to linearize, there's much less fussing than in previous years.  We just discussed the linearization, and for a grade, even.  Everyone paid careful attention (because they care deeply what grade they get on a quiz).  That doesn't mean everyone interprets their graph perfectly... but we're five stepping stones ahead of where we were in previous years, even though I've done fewer experiments in this style.  Quizzes work!

02 February 2015

USIYPT 2015 -- results, and problems for USIYPT 2016!


This past weekend, Woodberry Forest School hosted the 2015 US Invitational Young Physicists Tournament.  Nine schools from around the country and the world participated in "Physics Fights," ritualized discussions about research projects.  The teams included:


The Harker School, CA - CHAMPIONS

Woodberry Forest School, VA - Second Place

Rye Country Day School, NY - Final Four

Renmin University HS, China - Final Four

Nanjing Foreign Language School, China - Swartz Poster Session Champion

Pioneer School of Ariana, Tunisia

Shenzhen Middle School, China

Princeton International School of Math and Science, NJ

Phoenixville Area High School, PA

At the closing ceremony, the trophies are awarded, and then the teams are given their "homework assignment:"  The four problems for USIYPT 2016 were revealed.  

In 2016, the tournament will be held Jan. 29-30 at Randolph College in Lynchburg, VA.  Let me know if your school would like an invitation to participate, or if you would like an invitation to judge.  Problems include:

#1 --  Domino Toppling: On 6 August 2014, in Charlotte, North Carolina, a team from Prudential Financial broke the Guinness World Record for toppling the largest domino stone, measuring roughly 30 ft x 15 ft  x 3 ft.  Each domino in the chain had the same aspect ratio of 10:5:1.  Study this phenomenon, then design and construct a domino chain whose overall lateral length before toppling is 3 meters, that starts with a domino stone that you can hold in your hand, and will topple the tallest possible stone. You may change the aspect ratio of your domino stone chain, however all stones must have the same aspect ratio, and all stones must be constructed of the same materials and in the same manner. You must launch the initial, smallest stone with a gentle finger push that topples that stone.

#2 – Blender Lift: If you hold an immersion hand blender's blades under water in a beaker or pot or pail, under certain circumstances you can lift the beaker and the water by lifting only the hand blender as shown in the picture below.  Study this phenomenon for a wide range of the relevant parameters comparing your theory that explains the effect to the experimental results.  Predict the
maximum weight of water and container that your blender can lift and verify this prediction by experiment.


#3 -- Transformer Impedance Reflection: the recently posted YouTube video titled "Transformers – Experiments and Demos" (v=y0WrKT45ZZU) shows a demo at the 4 minute mark.  The demo purports to show that removing a light bulb in the secondary circuit of a transformer will cause a light bulb in series with the primary to turn off, i.e., "a impedance reflection." Analyze this demo and the published explanation of this effect (W. Layton  Transformer Impedance Reflection, The Physics Teacher 52 (7), Oct 2014, p. 426-427).  Provide theoretical and experimental evidence to explain or refute this effect.



#4 -- Bouncing Laser Beam: – a laser will curve and even bounce in a medium whose index of refraction decreases with height.  Although there are several ways to produce this medium, the photo below was created by pouring thick, transparent Karo syrup into a tank and then pouring water on top of the syrup.  Approximately 12 hours later, the bouncing laser beam can be observed.  Create this apparatus or a similar one, study the theory of this effect, and use your results to measure the index of refraction of the medium as a function of height from the bottom of the tank.