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22 July 2018

Mail Time: a tough mix for an upcoming AP Physics 1 class.

Nancy writes in:

I didn't have an AP class this past year because the numbers were too low. The class did make for this upcoming year, which starts in 8 days! I have 6 kids in my class. 3 had my honors physics class last year (and I feel have a good foundation for AP). The other 3 have never had physics. Any advice for how to start? Where? Any recommendations for folding in instruction for the newbies while reminding the trained without boring the one and losing the other (or me losing my mind)?

Yeek.  Looks like a tough mix... at least, though, there are only six students.  It's pretty easy to tailor the class with so few.  

I guess the answer depends most on how well prepared the three returnees are for the demands of AP Physics 1.  Is your honors class calculational, conceptual, or something in between?  

If your returning students had a calculational high school-level course similar to the New York Regents class, then I'd just teach the whole class AP Physics 1 together.  They'll all have to learn how to explain and write about physics.  Just be sure to begin the year with something clearly deeper and different from what they might have done last year; be sure to begin the year with a fast pace.

However, if your honors class was sort of AP Physics-lite, then you might want to do something further removed from a typical class.  Something like having returners teach the class occasionally, or assigning them as tutors for the new-to-physics students, or set up competitive lab exercises that partner returners with newbies, or... anything you can think of that makes the returning students the acknowledged leaders of the team so they barely even notice that they're covering the same topics again.

AP Physics 1 is intended as a first-year course.  In the long term, see if you can place more of your top-end first year students into the AP class.  That will allow you to cast a wider net for second-year students.  Even students with quite poor standardized test scores can do very well in AP Physics if they are seeing the topics for a second year, but that first year truly is not necessary for the majority of students.

Good luck...

14 July 2018

2016 AP Physics 1 problem 5 waves on a vertical string - video from Walter Keeley

In my PWISTA* AP Summer Institute, participant Walter Keeley tried to set up the waves-on-a-vertical-string problem from the 2016 P1 exam.  (Click the link, and go to problem 5.)  The problem asks students to explain, in a paragraph, how a picture of standing waves on a vertical string provides experimental evidence that the wave speed depends directly on the tension in the string.

*Putnam-Westchester Industry & Science Teacher Alliance

The correct approach:  The picture shows a longer wavelength in the standing wave near the top, where the tension in the significantly-massive string is greater.  Since the frequency of vibration is provided by the wave generator and is the same throughout the string, by v=λf the larger wavelength means larger wave speed.

Walter initially had no more success setting this up than I had over the years.  The strings I've used have simply not been heavy enough.  Walter tried a seriously massive rope, but it wasn't flexible enough to show a clear pattern.

It was Lorren, one of our participants, who suggested using ball-chain to provide both mass and flexibility.  It was Tom who pointed out that you'd have more luck seeing the wave if the generator didn't merely vibrate left-and-right, but in a small circle.

So, when Walter had some downtime at robotics camp, he obtained some chain (from True Value Hardware at 40 cents per foot).  Walter had to build his own geared motor to vibrate the chain:
Photo credit Walter Keeley

Walter used his phone to create a video of the vibrating chain; the app Hudl Technique is what he (and my students, too) use for playback, because it does slow motion and precise timing.

And oh my goodness, look at Walter's video: 

Look how you can see -- and measure, if you want -- the wavelength change.  Walter even arranged - presumably by trial and error with the frequency selection - to have the same three-and-a-half antinodes that are shown in the original AP problem.

Do you have a beautiful experimental setup of a released AP Physics problem?  Send pictures, and I'll be happy to post it on the blog with credit to you.  If you've never set up released AP problems in your lab, well, that's an awesome April open-ended lab assignment.

04 July 2018

2017 AP Physics 1 problem 1 with the draining batteries: we set it up

On the last day of my AP Physics 1 summer institutes, participants are asked to pick a released exam question, solve it... and then set it up experimentally.  We always get some simple yet elegant setups, like Rebecca’s modified-Atwood-turned-projectile from the 1998 exam; some complicated setups that don’t quite work but point in the right direction, like Lorren’s attempt at the 2018 collision-on-a-horizontal-spring problem which got us thinking about vertical springs and velcro; and some complex and elegant creations that go beyond just the AP question, like Quinn’s measurement of the kinetic friction coefficient that can then be used to determine the mass of an unknown block.

Several free response problems over the last few years have presented significant experimental challenges, though.  When the bumpy track problem came out on the 2016 exam I heard from so many teachers, "this isn't real, no one would actually set that up.  Well, we did last summer. 

This year one participant, Frank, took on an even greater challenge: the paragraph question about draining batteries, from the 2017 AP Physics exam.  It postulates connecting light bulbs to a battery in three ways, and asks which will drain the battery soonest.  

Which battery drains first depends on the power dissipated by the circuit: since power is energy per time, the shortest time to drain the battery will come from the circuit dissipating the largest power. By V2/R with the battery always providing the same voltage, whichever circuit has the smallest equivalent resistance will dissipate the most power.  So the bulbs in parallel will drain the battery first, followed by the single bulb, followed by the two bulbs in series.  That's the theoretical solution.

Frank and I and our lab assistant Tom brainstormed three approaches to this experiment.

(1) We could finesse the problem by simply observing brightness, which correlates directly to power.  That's almost trivial - we connected three sets of bulbs to three identical batteries, and we saw the parallel bulbs brightest, the single bulb next brightest, and the series bulbs dimmest.  Great for a simple demo, but this sort of thing has been done before.    

(2) We could get three fresh batteries, hook them up, and set up an iphone to do a time lapse video.  I don't know offhand how long it would take to drain fresh AA batteries, but it's not gonna happen within a single lab period; and that's all we had at our institute.  I do hope that some reader will set up the time lapse and send it along.

(3) Tom noted that he had some "big blue" capacitors available.  These are 27,000 microfarads and can operate up to 25 V.  Though the study of capacitors is beyond the scope of AP Physics 1, it's not hard to explain to students that these things store a fixed amount of energy (for a given starting voltage), and then that energy dissipates rapidly.  

How rapidly?  Your students don't need to know, but the time constant of discharge depends on the product of the equivalent resistance and the capacitance.  We want an equivalent resistance in the dozens of ohms to get time constants on the order of seconds.  Frank and Tom found some Christmas lights; when these were connected via alligator clips, the dimming of the lights over a few seconds as the capacitor drained was easily visible.

Which drained first?  It was sorta hard to tell with the naked eye.  So we clothed our eyes with slow motion video on our phones.  

Frank charged the capacitors.  He connected a switch that, when thrown, would simultaneously connect each circuit to its capacitor.  Here's the result:

Just like we predicted - physics works.