POTU Evolution Part 1: From California to Rio Americano

Encore Post: Originally published February 21, 2020—in the last days of The Before Times. Sigh.

UPDATE 2/23/20: Extant course chart added.

We plan to launch our Physics of the Universe (POTU) course in 2021-22. Physics of the Universe is the physics portion of the three-course model for Next Generation Science Standards (NGSS) implementation. California is where I live and work; Rio Americano High School is where I've taught physics since 1986.

Our plan is to discontinue Physics as it has existed at the school since its inception in the 1960s, and replace it with POTU. We will continue to offer AP Physics 1 and AP Physics 2 (when demand exists). I plan to retire at the end of the 2022-23 school year, so the course will evolve considerably after I'm gone.

Here is a pie-chart representation of the course as it exists, in terms of the unit topics.

But August of 2021 is coming, so there is a course to create. The POTU Evolution blog posts will chronicle my development process for the benefit of both of my blog readers.

The state of California has developed a framework for the three-course (Biology, Chemistry, Physics) implementation of NGSS. (Feel free to read it real quick. I'll wait.) Physics of the Universe is divided into six segments.

As this course-development project began, my first step was two turn those six segments into twelve units. And I felt I needed to move the nuclear unit. So already, some entropy is working its way into the pie chart. Notice that where I broke one segment into two or three, I maintained color fidelity to the framework's six segments.

As I read deeper into the framework, it became clear that some subsegments were more equal than others. As I began mapping out a day-to-day schedule, my concept of the course took this shape. It's not even trying to appear appealing anymore. But that's not what's important. There are 180 days to plan, and they're not going to plan themselves.

That's probably enough on the planning for now. I'll post unit schedules as I develop them. It's hard to overemphasize how drafty these visions are right now. But they are preliminary drafts at best.

A few additional details: the principal would like our POTU course to be accessible to freshmen. Given that our district is all-in on Integrated Math, any algebra necessary in the course will need to be taught in the course.

We will be adopting textbooks next year for all science courses. Our last adoption in physics was in 2008. POTU textbooks are... largely still in development. I think Conceptual Physical Science would work nicely for NGSS 3-course Physics and Chemistry, but we'll see what the adoption options are in 2020-21.

Thoughts? Ideas? Advice? That's what the comments section is for. I'm keen to hear about what you're doing as NGSS and new assessments appraoch.

Breakthrough: The Ideas that Changed the World

The Next Generation Science Standards place a new importance on engineering. There will be initiatives and ideas for how to engage students in designing and building things in class.

If we want to enmesh engineering into the fabric of our curriculum, we might want to consider shining a spotlight on the engineering history a few key inventions. The PBS series, Breakthrough: The Ideas the Changed the World shines that light on six such triumphs of engineering. The series played on PBS this past spring It streams on Amazon Prime. And the DVD set of the series, hosted by Patrick Stewart, is now available. Here's a rundown of the episodes.

The Telescope
Episode 1 tells the story of the development of the telescope, from a stone-age observatory to the space-based telescopes of the future.

It entails long-abandoned Stone Age dolmens once used as celestial calendars,
how Venetians made glass transparent, the optics of a medieval Persian camera obscura, a Dutch lens maker’s wartime breakthrough and a Venetian math teacher’s advancement of it, a Parisian invention improved by a spilled bottle of mercury, a team of women known as human computers who were armed with fly spankers, an athlete-turned-astronomer working high above Los Angeles, and telescope in space that will allow us to see as far as physics will allow.

Episode 1 "breakthrough celebrities" include Galileo Galilei, Edwin Hubble, Henrietta Swan Leavitt, Louis Daguerre, Hans Lippershey, and Ibn al-Haytham.

The Airplane
Episode 2 tells the story of the development of the airplane, from early human-powered attempts to the jet concepts of the future.

It involves a ninth-century moorish daredevil’s first attempt at human flight, how we had tails before we had wings, an artist obsessed with anatomy and flight, a whirling arm in the stairwell of a seaside mansion, the rubber band’s role in flight, the curve of a stork's wing, a connection to maritime technology and the gyroscope, the use of a deep-sea diving suit to fly high, why modern pilots are pressure tested, and the surprising efficiency of the jet propulsion gas turbine.

Episode 2 "breakthrough celebrities" include Leonardo da Vinci, Wilbur Wright, Orville Wright, George Cayley, James Doolittle, Wiley Post, Frank Whittle, Elmer Sperry, Lawrence Sperry, Otto Lilienthal, Alphonse Penaud, and Ibn Firnas.

The Robot
In episode 3, Locutus of Borg tells the story of the development of the robot. Well, Patrick Stewart is the series narrator, so... close enough!

It involves the ancient legend of Hephaestus and rudimentary Greek automatons,
a device that could learn a new tune and repeat it exactly, a desire to produce navigation tables by steam, the linguistic contribution of a Slavic cubist painter, the breakthrough of storage for programs, an imitation game, an electronic tortoise, a mission to Mars, a nuclear disaster in Japan, and the challenges of balance and hands.

Episode 3 "breakthrough celebrities" include Alan Turing, Charles Babbage, Ada Lovelace, Hero of Alexandria, William Grey Walter, The Banû Mûsâ brothers.

The Car

Episode 4 tells the story of the development of the car, from ancient sleds powered by primitive dogs to autonomous vehicles powered by graphene batteries. Gary Numan was not available for the narration, so Patrick Stewart handled the voice-over.

It involves ancient arctic sled dogs, the pairing the axle to the wheel, the smelting of metals from rocks, the spreading of a language, the need to pump water out of mines, the boring of naval canons, a stunt carried out by an inventor’s wife, the efficiencies of a slaughterhouse, and the promise of graphene.

Episode 4 "breakthrough celebrities" include Henry Ford, Karl Benz, Bertha Benz, James Watt, Thomas Edison, John Wilkinson, Thomas Newcomen, and Jay Leno.

The Rocket

Episode 5 tells the story of the development of the rocket, from ancient guano-powered fireworks to plasma rockets with magnetic confinement. 

It involves the use of bat guano from Chinese caves to drive off evil spirits, a modern-day celebratory rocket battle in Greece, a work of fiction by a famous astronomer, an imaginative tale about a trip to the moon, a visionary living in rural Russia, a breakthrough in dairy processing technology, a loophole in an international treaty, a repurposing of firefighting equipment, internal Soviet geopolitical subterfuge that stoked the Space Race, and plasma engines that could transport humanity to other worlds.

Episode 5 "breakthrough celebrities" include Robert Goddard,  Werner Von Braun, 

Konstantin Tsiolkovsky,  Jules Verne,  Ti Tian,  Sergei Korolov, Karl Gustav, Thomas Harriot, and Johannes Kepler.

The Smartphone

Episode 6 tells the story of the development of the smartphone, from the fall of Carthage to brain-interfaced apps of the future.

It involves ancient Roman battlefield communications, a painter-turned-inventor who devised a code still in use today, a groundbreaking technology whose rightful inventor had to be settled by the US Supreme Court, an obsessive inventor who found treasure in another scientist’s trash, a quirky keyboard talking machine, a Hollywood starlet who was also a prolific inventor keen to defeat the Nazis, the miniaturization of a circuit invented by an engineer who hadn’t yet earned vacation time, and a father eager to share a photograph of his newborn daughter with friends and family.

Episode 6 "breakthrough celebrities" include Alexander Graham Bell, Samuel Morse, Guglielmo Marconi, Hedy LaMarr, George Antheil, Jack Kilby, Polybius, Philippe Kahn, Homer Dudley, and Elisha Gray.

I produced companion question sets for each episode. They can be found at The Lessons of Phyz at Teachers Pay Teachers: Breakthrough—The Ideas the Changed the World.

CAST Reference Sheet: The Good, the Meh, the Bad, and the Wha?

With the NGSS-based California Science Test (CAST) set for its maiden administration this spring, a reference sheet has been prepared. Students have access to the reference sheet while they take the exam. Here's the reference sheet.

CAST High School Reference Sheet

Reference sheet development can awaken passions that physics instructors didn't know they harbored. So let's go through it. (It might be helpful to print the sheet or have it open on another screen. Blood pressure medication? A preemptive dose might save a life here.)

FORCES AND ENERGY
Good: Newton's Second Law, the weight equation, universal gravitation, Coulomb's law, gravitational potential energy, kinetic energy, work-energy theorem. All good.

Meh: Using w for weight. A little bit non-standard, but the judges will allow it.

MOTION
Good: F∆t = m∆v, the use of ∆v in the equation for acceleration: the ∆ is critical here.

Meh: The use of ∆t rather than t. We never create problems involving isolated clock readings (t); intervals (∆t) are always given. The ∆ in front of the x in the equation for speed? I can take it or leave it.

Meh: The use of J for momentum. That's a rare one. I've seen it, but it is not common. I prefer ∆p, and I don't think I'm alone.

Bad: s for speed. I am reminded of a line from Star Trek: "A Vulcan would not cry out so." In this case, I will turn that phrase to "A physics teacher would never, ever use s to denote speed. Ever." I hope I'm not being ambiguous here. The very use of s for speed makes me wonder if anyone associated with physics instruction was involved in developing this reference sheet. A math teacher might make such a miscue, but not a physics teacher.

Wha?: Using the Greek letter, rho (ρ), for momentum. Why? Lowercase ρ is a symbol used to represent density in high school or college courses. It is also used to represent resistivity. Nobody ever uses ρ to represent momentum. Anywhere, ever! Momentum is represented with the letter p. Extra effort is required to insert the letter ρ. I am dumbfounded.

KEPLER'S LAWS
These are here for the benefit of items addressing Earth Science PE, HS-ESS1-4. I'll let them "Meh" or "Wha?" the notion that e = f/d is among Kepler's Laws.

WAVES
Good: The wave equation and the Planck-Einstein relation. I would have put them in that order, but okay.

Wha?: The reference sheet is set in a sans serif font. In high school, we use lowercase f for frequency. Someone at CAASPP seems to have found the special character, ƒ, which bears a striking resemblance to an italicized, lowercase f. If a pianist sees two of those in a row (ƒƒ), they'll hit the keys hard. The symbol, itself, is called "F with hook," or "florin symbol" and is used to denote the Dutch guilder, for example. This has been Sheldon Cooper's "Fun with Fonts" with your host, Sheldon Cooper.

EXPERIMENTAL DESIGN
Mostly good. It it's me, I'd use a cross (×) rather than a bullet (•) to indicate multiplication. The bullet really isn't a multiplication symbol. The dot (·) is, but the cross is more appropriate here.

At the risk of belaboring a point made in the post critiquing the practice items, I will point out to our gentle readers that no reference to index of refraction appears on the reference sheet. No n = c/v and (thankfully) no Snell's law. Recall that one of the practice items expected students to be familiar with the meaning of index of refraction.

CONSTANTS
If someone at CAASPP sees this, please explore the use of the dot symbol ·. On my Mac, I hold down  the shift and option keys while typing a 9. It's more professional than the big bullet • that you get with option+8. And drop the spaces in unit configurations.

I see N·m as better than N • m for newton-meters.

In all, the reference sheet is more good than meh, bad, or wha? But I wouldn't be keen to print it up and have students use it throughout the year. Replace the speed s with a v and the momentum rho with a p, and I can live with the remaining quirks.

CAST Practice—An Analysis of the Physics Questions

The California Department of Education's California Assessment of Student Performance and Progress program has released a set of High School Practice Items in connection to the state-mandated exams to be administered this spring.

As detailed in a previous post, six of the 50 released items relate to high school physics topics. Sixteen are from Life Science, 15 from Earth Science, 9 from chemistry, and 4 from Engineering Design.

Remember: chemistry and physics topics have been combined into the more omnibus "Physical Science" realm of the high school science Performance Expectations (PEs). While NGSS's HS-PS1: Matter and Its Interactions is predominantly chemistry, HS-PS2 Motion and Stability: Forces and Interactions, HS-PS3 Energy, and HS-PS4 Waves and Their Applications in Technologies for Information Transfer are predominantly physics PEs.

Nevertheless, 9 of the 15 released HS-PS items assessed HS-PS1 while 6 were shared among PS2, PS3, and PS4. If there is a blueprint available for the composition of the operational exam, I am not aware of it. So the Practice Items may or may not reflect the mix of the operational exams.

UPDATE: I found the blueprint: for the high school test, see page 8 of the CAST Blueprint. Not surprisingly, it appears the operational exam will include physics items in greater proportion than in the released practice items.

Let's take a look at those six. I will state each item's Item-Level Claim Statement (ILCS). Click the ILCS to see the actual item. My analysis will follow each ILCS.

---

Identify the relationship between mass and acceleration. [Click to see item.]
In general, the item is perfectly reasonable. It speaks to the interpretation of graphed data obtained through a laboratory activity that might be done in a high school setting. The item links solidly with the corresponding PE (HS-PS2-1: Analyze data to support the claim that Newton’s second law of motion describes the mathematical relationship among the net force on a macroscopic object, its mass, and its acceleration.).

But there is a problem. And I say this as someone who had the privilege of sifting through many, many potential exam items offered by ETS for use in state-mandated testing from 2003 to 2013. The problem is the title of the graph.

Classroom teachers come across as dictatorial joy-crushers when they make any attempt to direct their students to conform to established conventions. Titling a graph is but one such challenge. The title of a scientific graph is Dependent Variable vs. Independent Variable.

The graph on this item is correctly constructed as Acceleration vs. Mass: experimenters would have measured the acceleration of a cart while varying the cart's mass. But the graph is titled Mass vs. Acceleration. I have to assume this was an oversight on the part of ETS and anyone (if there was anyone) tasked with content review.

---

Mathematically determine the properties of the system using the conservation of momentum of objects in the system.
A fair enough item to assess the corresponding PE (HS-PS2.2: Use mathematical representations to support the claim that the total momentum of a system of objects is conserved when there is no net force on the system.).

Some might object that the mathematics is simplified too much in this item. It does leave open the question of how deep the arithmetic might go in operational items. If you make this question about a neutron and a deuteron combining to form a triton, the principle is same, but the numbers are more formidable.

In the event that anyone tries to champion the new assessments as ground-breakingly novel, consider this released test question from the old Academic Content Standards era: Conservation of Momentum RTQ. In fairness, the new one does have color. And kilograms, rather than tons!

But this brings up an ongoing problem suffered by standards exam item-writers. Standards (now PEs) are often important-sounding principles that turn out to be difficult to write a variety of items for.

---

Select the design solution that best meets the provided criteria about momentum and force during a collision.
This item reveals the challenges associated with writing questions for PEs like HS-PS2-3: Apply scientific and engineering ideas to design, evaluate, and refine a device that minimizes the force on a macroscopic object during a collision. The item has a very long stem leading to the answer choices. And the choices are quite word-heavy, too.

Students with a firm grasp of the fact that increasing impact time reduces impact force will find their way to the correct answer. The item will make them work for it, though. I'm not faulting anyone here: awkwardness is in the nature of writing items aligned to standards (PEs) like this.

---

Create a correct mathematical representation to determine the components of gravitational potential energy in the Earth-ball system and kinetic energy.
This one requires a written response and is graded with a rubric.

It is linked to HS-PS3-1: Create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.

Might it connect more directly to HS-PS3-2: Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative position of particles (objects)? The clarifications on PS3-2 say "Examples of phenomena ... could include the conversion of kinetic energy to ... energy stored due to position of an object above the earth. Examples of models could include diagrams, drawings, descriptions, ...]

Aligning items to PEs is sometimes a dark art; I'm inclined to grant CAASPP latitude on this matter.

I have reason to suspect Rhett Allain will not approve of dropping a tennis ball from the top of a building and stating that air resistance is negligible. Neglecting air resistance is a simplifying step. Dropping a tennis ball might be a requirement of item-writing guidelines that prohibit scenarios that might cause significant injury. Dropping a cannon ball would inherently render air resistance more negligible, but would also present a greater potential (!) for damage if carried out in real life.

There's some contrivance at work here: Determining the kinetic energy of a dropped ball in reality would involve measuring its mass and its speed. The speed, alone, allows one to determine the height from which the ball was dropped. But the standard demands items, so here we are.

---

Describe how wavelength is related to the change in the medium.
The PE being addressed here is HS-PS4-1: Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media. The Disciplinary Core Idea expands this to: The wavelength and frequency of a wave are related to one another by the speed of travel of the wave, which depends on the type of wave and the medium through which it is passing.

One word that doesn't appear anywhere in the HS-PS PEs is "refraction". So we have a judgment call here. Do we assess attainment of this PE by asking about what does or doesn't change at an optical boundary during the process of refraction?

Students must also know what index of refraction refers to and that greater values indicate slower transmission speed in transparent materials. I'm delighted to teach the material; no one gets through my course not knowing how rainbows work. But I might have omitted index of refraction thinking no harm would come to my students based on a strict reading of the PEs and DCIs. I would have been wrong.

There are certainly topics I am skipping based on my reading of NGSS. Which ones will show up in the assessments? Time will tell. I'm not a huge fan of surprises like this.

Minor point: Numerical values were used for the indices of refraction and for the wavelength of the laser. I'm not sure why an infrared wavelength was chosen. When I use lasers in class, I prefer to stick to visible light varieties.

---

Quantify the change in energy associated with the appropriate change in the relative orientation of the two objects.
The item is a pretty spot-on assessment of the PE here, HS-PS3-5: Develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction.

I like this one because it shows a weakness in my own instruction that will require a bit of patching up.

During classroom instruction, students should be able to examine a number of systems and understand how potential energy changes within them. A rock and the Earth, two opposite charges, two like charges, magnets, springs... Students should know where potential energy is zero and where it is maximized in a given system.

I'll need to work on more explicit instruction of that beyond gravitational systems.

---

If you made it all the way to the end, pat yourself on the back. This was a long one. I will nourish a hope that we get more released items each year. I was unambiguous about this priority when I served on California's Assessment Review Panel. But panelist's wishes were not always accommodated. I really am trying to be subtle here! In any case, I will try to maintain cautious optimism.

Having said that, I will add that the most fun I had practicing and honing the craft of teaching physics occurred in the years when my students were not assessed with end-of-course exams intended to enforce a measure of accountability. My students in that era did not leave my course with woeful gaps in their physics knowledge. But that's just me shaking my fist at the sky.

Roadmap to the End of Physics

I believe the first time Physics was offered at Rio Americano High School, it was for the 1966-67 school year. My now-retired former colleague, Marion Gribskov taught the course for 19 years until he was called upon to teach Chemistry in 1985-86. I began teaching Physics in 1986. Based on my current reading of the tea leaves, I will teach Rio's final Physics classes in 2020-21.

In 2021-22, I will begin teaching Physics of the Universe (PotU). This will be our school's adaptation to the Next Generation Science Standards and the California Science Framework's digestion of NGSS.

As with most transitions of magnitude, there will be some awkwardness. I have penciled out a roadmap that will take my school from where we are now to where we need to be in 2023.

Here's the roadmap.

Here are the accompanying notes, for what they're worth.

PotU/11-12: Physics of the Universe for Juniors and Seniors. “Algebra 1”-equivalent prerequisite. These juniors and seniors will have had the opportunity to enroll in Biology of the Living Earth and Chemistry in the Earth as freshmen and sophomores.

PotU/11-12 might be a transitional course for use during Rio’s migration to the NGSS Three-Course model. It will be taught in 2021-22 and 2022-23 only. For 2023-24 and beyond, PotU will be a 9-10-level course. PotU/11-12 might be retained to accommodate juniors and seniors who did not enroll in PotU/9-10 but aren’t up to the challenge of AP Physics 1.

PotU/11-12 will omit physics topics not included in the California Framework Physics of the Universe course model, and add earth science topics as needed.

PotU/9-10: Physics of the Universe for Freshmen and Sophomores can omit the math prerequisite but will need to integrate some elements of algebra instruction.

Concurrent offerings of PotU/11-12 and PotU/9-10 in 2021-22 and 2022-23 are to prevent classrooms populated with freshmen through seniors. We put an end to this practice with Earth Science in 2016 with the introduction of Conceptual Physics. Earth Science was restricted to freshmen and sophomores while Conceptual Physics was restricted to juniors and seniors..

Development of Rio’s Physics of the Universe courses will be undertaken during all available collaboration time in 2019-20 and 2020-21 and additional time as needed and approved.

*Beginning in 2022-23, the prerequisite for AP Physics 2 will be “successful completion of AP Physics 1.” [The previous prerequisite had been “successful completion of Physics or AP Physics 1.”]

**Beginning in 2023-24, PotU/11-12 will be discontinued and PotU/9-10 will be the sole non-AP Physics course.

It should be noted that my district has not adopted physics textbooks since 2008. While the district has opted for the three-course model, it doesn't seem any textbook/curriculum program exists for this new course.

It seems there is an assumption among NGSS visionaries that they have provided a grand vision, and now it's up to classroom instructors to develop the curriculum that will implement their vision. That's simply not going to happen. I'm delighted to develop curriculum... to implement my own vision of what high school physics instruction should be. Anyone else who has a vision needs to provide their corresponding curriculum if the hope for implementation.

Between now and then, things may change in terms of the process. This is version 1.0 of how I think it will all go.

Practice items for California's upcoming NGSS test

The time has come for students of science in California. That's right, CDE's CAASPP's NGSS CAST from ETS goes operational this spring.

That wasn't a cat walking across my keyboard; that was the initialisms and acronyms that spell out the new state-mandated assessment regime in California.

NGSS is, of course, the Next Generation Science Standards. CAASPP is the CAlifornia Assessment of Student Performance and Progress. CAST is CAlifornia Science Test. (I'm suddenly envious of the science tests given in Missouri and Virginia, less so for those in Colorado, Delaware, Georgia, Hawaii, Maine, Nebraska, Rhode Island, Washington, Wisconsin, and lastly, Louisiana. ETA: Looks like I missed Michigan and passed Pennsylvania. I'll be here all weak.)

NGSS has been around for some time now, with its multi-colored documentation of Performance Expectations, Science and Engineering Practices, Disciplinary Core Ideas, and Crosscutting Concepts. California has digested NGSS into its own Science Framework. The Framework and NGSS articulate broad, sweeping visions of science instruction.

CAASPP is the program tasked with testing. The previous iteration of this (in California) was State Testing And Reporting (STAR). This is where the lofty visions presented in the vision documents (NGSS/Framework this time) must be broken down into test questions ("assessment items"). As it was during the STAR era, the California Department of Education (CDE) has contracted the services of Educational Testing Services (ETS) to develop the CAST. (The proliferation of initialisms and acronyms indicate the importance of the enterprise.)

Assessments are where the rubber of the grand visions meet the road of perceived accountability. I say this as someone who served on the Golden State Exam development committee and was appointed by the State Board of Education to serve on CDE's CRP (Content Review Panel, later rebranded as the ARP: Assessment Review Panel).

I am a relentless advocate for released test questions (RTQs). The vision documents are necessary, I suppose. But they never really specify measurable outcomes. The visions boil down to "all students should be knowledgeable in science and capable of performing scientific analysis and related tasks." But vision documents quickly blow up into phonebook sized documents with webs of interrelated objectives that can make tri-level chess seem simple by comparison. But with a sufficient bank of RTQs, I will know exactly what your vision was. Inductive reasoning is really the only way to connect standards and assessments, here.

Until recently, there were just a handful of CAST RTQs available. You can find them on the California Science Test Training Items Scoring Guide: High School (PDF).  The mix of topics (PE/DCIs) left much to be desired.

Life Science: 6 - Earth Science: 1 - Physical Science: 0 - Engineering Design: 0

Remember that in NGSS, high school science opposes three domains: Life Science, Earth Science, and Physical Science. The old subjects known as "chemistry" and "physics" have been combined into Physical Science. But not even one RTQ covered anything in Physical Science.

More recently, a raft of 50 RTQs has been launched. You can find them on the California Science Test Practice Items Scoring Guide: High School (PDF). The mix seems a bit better:

Life Science: 16 - Earth Science: 15 - Physical Science: 15 - Engineering Design: 4

Of the 14 items tagged as Physical Science, 6 related to topics covered in physics. The other 9 were chemistry questions.

Comments and criticism of specific items will be offered in a later post.

Crushable Concrete & Impulse

I find myself repeating "Longer time, smaller force," throughout my momentum unit. There are so many examples of safety devices that decrease the force one might experience by increasing the time. Bike helmets, car bumpers, crumple zones, air bags, seat belts, etc. All decrease the force experienced by increasing the time of the collision. The sticking point is always that the impulse is the same regardless. Many of my students incorrectly think that by increasing the time they have somehow managed to decrease the impulse. I remind them that the vehicle is going from 60 mph to 0 mph whether they use their brakes or hit a wall. They also seem to struggle with applying this concept to larger objects .... And what's larger than a jumbo jet?

Awhile back I caught a news segment about an airplane crash in which an Engineered Material Arresting System (EMAS) safely stopped a plane. Apparently pilots overshoot the runway sometimes and, well, its hard to stop something as big as a jumbo jet. This Popular Mechanics article gives a good background: "EMAS is essentially a rectangular bed of 2,000 to 4,000 collapsible cubes glued in place at the end of a runway, nearly level with the ground. As a plane careens into the cubes, the cubes break apart. Friction between the cubes and the plane's wheels ultimately slows the plane to a stop." The article continues to say "The system can safely stop a Boeing 737 traveling at 65 miles per hour in fewer than 300 feet."

Oooh, that sounds like a Physics problem! All you need is the average mass of the airplane and students could calculate the size of the force that EMAS applies to stop the plane. According to the FAA Fact Sheet for EMAS all runways need an extra 1000 feet past the end of the runway for emergencies. I would be careful that students don't confuse stopping "slowly" through 1000 feet with airplanes that often crash roughly at the end of a rough, unmaintained dirt patch that happens to be 1000 feet long. The point of the EMAS is stop in a short distance in a long time, compared to stopping quickly by say hitting a wall or a long rough distance over a long time.

Since there are many different possible stopping situations solving problems about this with students should be accompanied by simple diagrams, maybe even descriptions for each. If  you don't want to get into practice problems you can at least show students pictures and videos of airplanes stopping with EMAS.

I plan to bring up this material in my Crash Cushions project (original post here, additional information about leveling here). In my initial tests of the project a few years ago I found that small paper cubes were the most successful in minimizing the force, a similar design to EMAS!
Bringing in real-life examples that students can evaluate and analyze can help them improve their designs which is an aspect of the NGSS Science & Engineering Practices.


An explanatory video from the company:

Lecture replacement fail & success

As part of our NGSSification this year I've tried to put more demonstrations into the hands of students. Before the beginning of each unit I will look at the lectures of previous years and the list of demos I plan to do. If at all possible, I will make miniature versions of the demos that students can do in lab groups. These miniature versions don't usually take long but engage the students. We can then follow up with a few brief notes that explain what they have seen.

At the beginning of our light unit I wanted students to explore the relationship between wavelength and frequency in the electromagnetic spectrum. I made a chart of eight electromagnetic waves, places they are seen or used, a wavelength and frequency (pdf or google doc). I printed the information onto address labels (pdf) and put them on blank cards from RAFT. I passed out a card to each student and asked them to find the three other students that have cards about the same electromagnetic wave. Students were to find their group and then share their information so that they got the "whole picture" for that particular type of electromagnetic wave. I asked students to look at an image of the electromagnetic spectrum on my projector and a different one in their textbook. I thought that between the two images and four students they would be able to match the name to the example to the wavelength to the frequency. I was wrong. Quickly it became apparent they didn't see the connections I did.

Students were looking for the exact frequency on their card to be listed on either the image in their textbook or on the projector screen. Instead images of the electromagnetic spectrum show a range of frequencies for different types of waves. It is very common for sources to have different frequency ranges for the same wave type. This lead to lots of confusion and meant I had to go from one group to another to make sure that each student was sorted correctly.

In the future, I'll only show them one image, the one below. I added red dots representing each frequency and wavelength combination that I had put on the cards. I'll tell students that if they use the wavelength equation they will find someone with a card with the exact frequency they calculated. It was also not obvious to students that they needed to use this equation. I'm hoping this illustrates to students that their specific wavelength is part of a range of wavelengths for that type of wave.

After students were properly sorted I asked them to write the information about their wave onto a whiteboard to share. The whiteboards were held up and shared with the whole class. I helped students organize the information in a more familiar electromagnetic spectrum. To make sure they got the correct information it became more "Write down what I'm saying," and less "Write down what was written down by your peers." By the end of it I feel like they understood the relationship between frequency and wavelength but it was not the clearest way for them to get the information. I don't know if I will continue this activity the same way again.

The second day I wrote an activity I called Light Phenomenon Lab (google doc or pdf with more teacher information) made of eight different demos I used to show as part of a lecture. Each group was given brief instructions about how to create the phenomenon to observe and given some time to play. There were great exclaims of "Oh cool!" and disbelief as students explored and shared their phenomenon from group to group. Some groups were asked to read a page or two out of their textbook that explained the higher level concepts.

Each group was instructed to make a whiteboard with the title of their phenomenon, what they observed, how it worked, etc. Each group had been asked a question that required them to apply what they had seen to a new situation. This question was to be answered on their whiteboard as well. Below are examples from my three classes:

Each student copied down the phenomenon name and the answer to the question posed after each group shared their board. This was much more successful than the previous day. Students took ownership of being the "experts" on their phenomenon and enjoyed wow-ing the rest of the class by showing it off. It lead to more questions, "playing" with the equipment to find out what they could discover. I was able to create a make-up lab (pdf or google doc) for students using YouTube videos of similar phenomenon.

On both days, students were hands-on with something or moving around. We still discussed a few things and I asked them to record some things in their notebook. One I would consider a #fail (or in need of some tweaking) but the phenomenon day I would definitely consider a success. Students still got the important elements of the "lecture" but it was much more engaging.

Soon I'll post more about NGSS Phenomenons ... after Finals Week.

Is there gas in that light bulb?

I'm a fan of topics in thermal physics. Not thermodynamics as much as temperature, heat, and heat transfer. These topics have been largely abandoned by NGSS (HS-PS) and are not included in AP Physics 1. For the moment, I teach AP Physics 2, which has some concern for thermodynamics. That gives me reason enough to teach thermal physics as a precursor.

We cover that content in the fall. We are now into geometric optics. That's how it was that I came to have a 40-W incandescent bulb with a dimmer switch on a gooseneck lamp glowing on a classroom table. I was demonstrating the classic image-formation trope of "What will happen the to the image if half the lens is covered?" (It's right up there with "How would the results of this [mechanics] experiment be different if it were conducted on the moon?" among well-worn item-writing chestnuts.)

But we had a few minutes before the end of the period. So I offered a question: "How could you know whether or not there's gas in this bulb?" If I had planned this inquiry a bit better, I would have  begun with "Why is the filament enclosed in a glass bulb?" If incandescents weren't becoming increasingly rare, we could break the glass on one and see what happens when the filament is exposed to air.

Of course, the immediate solution posed by my thoughtful teenage (mostly male) scholars—after milliseconds of contemplative deliberation—was "break it!". You hardly need a question. The best and most immediate answer is going to be "break it!". It just is.

I asked how breaking the bulb would reveal the answer to the question. Forced into a corner of their own making, they suggested weighing the bulb before and after the break. A difference in weight would reveal the prior existence of the gas. I argued the difficulty of the logistics and the precision required.

To move them off the property destruction solution, I moved the goalposts. How could you know without breaking the bulb? And without any other instruments? Hemming and hawing ensued. The end of the period was approaching.

Is the glass strong enough to hold up under atmospheric pressure if it's evacuated? Maybe. Could there be a gas in there given the rapid burn-out that happens when the filament is exposed to air? Yes: inert gases, noble gases.

I touched the bottom of the bulb and reported that it was warm. I casually kept my fingers on the glass of the bulb. Didn't the transfer of heat from filament to bulb require a conducting medium? No, they insisted. The bulb could have been warmed by thermal radiation.

An inquisitive student got up and touched the top of the bulb. He did not keep his fingers on the bulb very long at all and complained about how hot is was. (I had adjusted the 40-watt bulb down to about 10 watts, so it wasn't as bad as it could have been. I also assured him any burns would heal in a few days.)

At that point, they got it. The bottom was warm but the top was hot. Gas in the bulb is heated by the filament, rises, and deposits heat on the top of the bulb. Convection!

The key to knowing there was gas in there was feeling the top and bottom of the bulb. Having the bulb in a horizontal orientation helps: you have a "top" and "bottom" made of the same glass. The FLIR One thermal camera image was an afterthought, and is not needed to develop a solution.

When the period was over and students were filing out of class, the inquisitive student told me that breaking the bulb could be a simple solution, as long as you broke the bulb underwater.

He had me there.

How many magnetic poles?

Last weekend I presented at the Exploratorium's 4th Annual NGSS STEM Conference "Making Science Count: Integrating Math into an NGSS Classroom." I presented a few inverse and inverse square relationships participants explored using hands on experiments. One of them was to investigate the relationship between the strength of a magnetic field and the distance to the object.

Thanks to sponsors, participants were able to go home with their own "cow magnet" (if you don't know why they are called that read about Hardware Disease). While preparing for the workshop that morning senior scientist and staff physicist Paul Doherty cautioned me that while I would expect cow magnets to be dipoles they could be tripoles. After he check with magnetic viewing film it turned out they were quadpoles. And that can complicate an experiment.

Workshop participants either borrowed my Vernier Magnetic Field Sensor or used the magnetometer on the Physics Toolbox app during the workshop. When I was preparing for the workshop I found that this could be an inverse square or an inverse cubed relationship depending on the physical dimensions of the magnet. Given the orientation of these quadpole magnets if you rotated the cow magnet at all as it approached the sensor the polarity could change.
1. Asking questions (for science) and defining problems (for engineering)
Below is a video using a dipole donut magnet, a dipole cow magnet and a quadpole cow magnet that models what I would expect students to see.

I investigated further using my Vernier Magnetic Field Sensor once I got back to school. Below is a graph made by starting the sensor perpendicular to one end of the cow magnet and then moving up the length of the cow magnet to the other end. I put a pencil in between the cow magnet and the sensor to maintain the same distance between them.

On the left, the red line was made using a dipole cow magnet and the blue by the quadpole magnet. In this case they are similar and one might conclude that they are both dipoles. (Differences in slope are due to the sensor's speed.) On the right, the red line is the same, made with the dipole magnet. The orange and green were both made with the quadpole magnet. When the green line was made the magnet must not have had a pole directly facing the magnetic field sensor.

I also pointed the sensor at the end of the cow magnet and rolled it along the table, keeping the sensor from rolling and at the same distance away. I tried it twice with the quadpole magnet, creating the green and purple lines in the middle. This was harder to keep steady but you can see the polarity switch as the lines pass the time axis. Repeating the experiment with the dipole created the brown line at the top of the graph. For the dipole rolling it made no difference in the polarity strength or direction. 

So what can you do with a pesky quadpole cow magnet? Why, confuse your students of course! I plan to hand groups of students one dipole and one quadpole cow magnet and ask them to determine the number of poles on each. If they are lucky they will get a compass and/ or viewing film. Otherwise having two magnets should be interesting enough. If you're keeping track of the NGSS Science & Engineering Practices, such an investigation could lead to quite a few of them in one lesson:

2. Developing and using models [of thinking]
3. Planning and carrying out investigations
4. Analyzing and interpreting data
5. Using mathematics and computational thinking
6. Constructing explanations (for science) and designing solutions (for engineering)
7. Engaging in argument from evidence
8. Obtaining, evaluating, and communicating information

Analyzing a simple Crash Cushion design

When my students make their crash cushions many of them create paper tubes. This can lead to a successful design or a horrible one. I saved one such simple design and tested it in three different orientations. Depending on the level of your students you could show them this to aid with their own design development.

Crash cushions cont.

Several years ago Dan Burns and I started discussing an engineering project for which students build a crash cushion to investigate momentum and impulse. Using only a few sheets of paper and hot glue student groups design crash cushions (similar to water barrels or guard rails on roadways) to lower the force experienced by a cart rolling down an incline that crashes into them.

Since then we have both completed the project with classes albeit differently. We also presented at the Summer 2016 AAPT meeting in Sacramento, all of the materials discussed there are here. I wanted to share the project here again, with the tweaks my partner Jon Brix and I have made to it since.

My colleague Matt Miller continued the project in Conceptual Physics this year although opted not to use the Vernier sensor that I had last year.  He opted for the resettable Drop N Tells I bought years ago instead as it is more visual for the younger students. He set up a ramp and used a lightweight impact car that had an additional <200 grams of mass added. Miller adjusted the ramp set-up until the 25, 15, 10 and 5-g sensors were consistently tripping. His students were challenged to design the crash barrier that did not trigger all the sensors. I believe he set the grading up this way:
C = triggering the 15, 10 and 5-g sensors
B = triggering the 10 and 5-g sensors
A = triggering only the 5-g sensor
Extra Credit earned for not triggering any sensors.

Dan uses a PASCO Smart Cart while I use Vernier sensors. The first year I tried this I used a low-g accelerometer because it was what I had. Through a Donors Choose grant I was able to purchase the higher 25-g accelerometer, 3-axis accelerometer and a Wireless Dynamic Sensor System (WDSS). The wired sensors require some coordination to prevent the cord from catching but are workable. I found that the wireless WDSS made for easier set-ups but would disconnect occasionally. Both the WDSS and the 3-axis sensors were almost too accurate and the graphs produced were difficult for students to interpret. I opted this year to use the single-axis 25-g accelerometer because even collecting 500 samples per second the peak accelerations were easier for students to determine.

In the past I've used a wood ramp and a big heavy dynamics cart that then travels along the flat lab bench into a wall. The transition from ramp to flat tabletop caused additional acceleration peaks so we opted to have the cart run directly into the wall from an incline. The heavy cart and steep ramp produced a high acceleration that exceeded the accelerometer's limits. We decreased the ramp angle and still occasionally "missed" the hit because the time of impact of the cart against the wall was so short. This year we opted to use a low-friction (not smart) PASCO cart and track from another colleague. The 120 cm long track was raised above the table by one textbook and pushed against the wall. A box of weights (over 30 lbs) was pushed against the higher end of the track to prevent it from moving. In initial tests the conservation of momentum caused the track to move quite a bit when the cart struck the end of the track.

This year student designs proved very successful. Because of the light cart and small incline students were able to reduce the acceleration of the cart at impact fairly easily. Usually the designs that "failed" did so because the cart passed underneath the crash cushion and still struck the wall. I had only one set-up in the classroom so groups took turns testing their barriers and collecting data. We stored a trial of the cart running into a book at the end of the ramp and then printed out graphs for each group with their trial on top of the control data. Here is an example of the data student's received with the control (green) and their trial (blue):

Students were to take measurements of their crash cushion before and after their collision although most cushions did not permanently deform. For some reason students were very pleased when their crash cushion suffers little damage; several cited the fact that it could be reused as a positive attribute. Students were also to calculate the Force with and without their crash cushion based on the mass of the cart.

Using this information students were to write a Claim, Evidence, Reasoning (CER) conclusion to answer the question: "Was your crash cushion effective?" After grading these conclusions we realized a few things:
1. Students did not agree on what made a crash cushion effective. Most students realized that decreasing the force, as shown on their graph as a decrease in acceleration, by increasing the time of impact made for a successful crash cushions. A few more realized that stopping the moving cart without letting it bounce back was also good. Yet many students considered their crash cushions ineffective if there was any acceleration, even if they reduced their force by more than 50% .
2. Students do not know what is fact vs. opinion. This must be going around recently. Students often stated opinions or qualitative observations in place of specific measurable data. "Our crash cushion was good because it stopped the cart slowly."
3. Some students did not understand the graph axis, significance of peaks, etc. Referring to the example above, some students incorrectly described the "time of impact" to be just over 2.5 seconds for the control trial. They did not understand or forgot the fact that the cart had to roll down the ramp before the impact.
4. When in doubt, students are prolific. I expected three, maybe 5, sentences from students yet often received a full page. While grading these conclusions I often crossed out over half of what was written because it was superfluous. They seemed to just keep writing and praying for partial credit.

Before handing back their conclusions I reviewed the CER format with students and showed them a few pictures of correctly written (short) examples of their peers. I showed them a few sample graphs from their trials and reviewed the significance of each peak. In the future this will be done the day after to give students a chance to correct their CER conclusions before turning them in.

Roll the dice

Studying the Law of Universal Gravitation can be heavy (ba dum tss) for students. Each year my students push through the long equations and we go without a lab for about a week. That's pretty unusual in my classes and they can feel the change. If a student asks why we aren't doing a lab I usually reply, "Well I can't haul Jupiter in here to measure it so ...."

Practice problems were always difficult for students, more about living by the Order of Operations (PEMDAS) then actually understanding the problem. Some would take one look at that period of revolution equation and say "No, nuh uh, not gonna make me. Nope."

I tried to make them fun by creating word problems. It wasn't just calculating the Force of Gravity between you and Jupiter, "Let's compare that to the Force of Gravity between you and the doctor that delivered you! Jupiter's gravity doesn't affect you and astrology is bunk!" But still going through problems together wasn't engaging students.

Your birth Mass (kg)	Jupiter & Doctor (kg)	closest distance (m)	Universal Gravitational Constant	Force of Gravity (N)
4	1.90E+27	588000000000	6.67E-11	1.47E-06
4	100	0.1	6.67E-11	2.67E-06

A few years ago I had an idea to make our practice calculations into a dice game. One di has problems to solve and the other different planets. There are actually two different planet di so that students can choose. Here is a print ready file, I suggest thicker paper to give it some structure. They take some time to assemble but you can use them for years to come.

The kids loved it. They probably work through more practice problems than they would have if I had just supplied them with certain ones to do and they were very invested in letting chance choose which they would calculate. A new development this year was that they are so used to checking their answers with me while they whiteboard they wanted to know the "answer." That was harder to do as I walked around the room since there were so many variations. I decided to create and project an answer chart in Excel.

Depending on how your school or district is defining "physics" from the NGSS framework you may or may not be finding yourself teaching more Earth & Space Science. We had our own sort of NGSS Draft among Chemistry, Physics and Biology trying to divide up the Earth & Space Science topics. In my district Physics ended up, rightly so I feel, with HS-ESS1-4:

HS-ESS1-4.

Use mathematical or computational representations to predict the motion of orbiting objects in the solar system.[Clarification Statement: Emphasis is on Newtonian gravitational laws governing orbital motions, which apply to human-made satellites as well as planets and moons.] [Assessment Boundary: Mathematical representations for the gravitational attraction of bodies and Kepler’s Laws of orbital motions should not deal with more than two bodies, nor involve calculus.]

This goes well with HS-PS2-4 about the Law of Universal Gravitation:

HS-PS2-4.	Use mathematical representations of Newton’s Law of Gravitation and Coulomb’s Law to describe and predict the gravitational and electrostatic forces between objects. [Clarification Statement: Emphasis is on both quantitative and conceptual descriptions of gravitational and electric fields.] [Assessment Boundary: Assessment is limited to systems with two objects.]

I'm still working on adding more of Kepler's Laws into the curriculum, there is an outline here on this Disciplinary Core Idea about it.

Evolution of Physics Curriculum

As I NGSSify my curriculum I find myself removing some pieces of my curriculum that I've done for a long time. Sometimes they are replaced with better activities, sometimes they were dead weight, sometimes I'm sad to see them go. I'm not there yet but it is a process for which we still have several years as the NGSS assessments might be ready by 2018. You've got some time.

While making these curriculum choices I ask myself:
- Does this activity align with NGSS in content (addressing a Discipline Core Idea or Performance Expectation)?
- Or does it align with the Science & Engineering Skills?
- If not is it an essential skill to support NGSS acquisition? (i.e. graph making)
- Is it content that they need to support late NGSS content? (i.e. Newton's 1st and 3rd Laws)

As we approach the middle of the 2nd quarter I find that I am "behind" my past self by almost two weeks. This has meant moving Energy, Work & Power to second semester rather than cram it in first semester. My time crunch is due to a few big changes in my classroom that are still getting the kinks worked out:

1) This year I do not have required homework other than finishing labs. There are some "suggested homework" each night and the problems that I really like are worked into the class period. I suggest that struggling students do the homework each night, it is very briefly reviewed each day because less than a quarter of the students do it nightly. A lot of my students are in more than one AP class, sometimes I am their only non AP class, so if they are understanding the material they don't have to "waste" time doing homework if they don't need it.

2) So how do students know they should try the optional homework? Each week students take a low-stake weekly quiz based on the homework. That way they and I know how they well they are understanding the concepts. I grade their notebooks at the same time so that I can look over their labs for the week. Timing has been an issue so far. Sometimes the quizzes are too long and take most of the period. Absences have also delayed getting the quizzes passed back so in the future I think I will excuse the absent kids. This mean I don't have a chance to see how they are doing before the big assessment and the other quizzes in that category count more but it should improve the pass back time. I want students to have near immediate feedback on these formative assessments. I have been crudely tracking the standards addressed in each quiz in an Excel sheet but they have not been in a way that can be easily shared.

3) Students complete problems on whiteboards in small groups. This has been successful but sometimes not universally so depending on a few things. Some students dominate whiteboards just as they do labs so now I ask students to pass the marker on after each problem. Some students don't feel confident enough in their abilities to problem solve while others are watching. If the group is motivated to be "done" as fast as possible then they miss out on the conversations and growth to get something written ASAP. Yet some of the best problem solving think-out-loud collaborating discussions my students have ever had have taken place this year. They correct each other by citing previous activities, "Remember when she said this? Remember that one lab we did?".

Overall I like the changes and will be keeping them, with some revisions. I need to focus on making sure the weekly quizzes tackle common misconceptions just as much as calculation practice. I need to emphasize and normalize doing the suggested homework without making it seem required. It should feel like an opportunity for students; I want struggling students to want to do it to improve themselves. For whiteboards we have to set up community expectations that include all the students of the group at once and makes it a safe space for all of them to try, make mistakes and try again. And I have to work on my timing by probably further cutting some material. But that will be a topic for another post ...

Science & Engineering Practices poster

The Synopsis Outreach Foundation Sciencepalooza has worked with a graphic artist to create a visual of the NGSS Science & Engineering Practices poster. Its a great addition to your classroom and available here. A preview is below, I suggest you download it!

PVC Dart Dun Lab tips

I've written about making simple PVC dart gun shooters and how to use them in the classroom with NGSS. I just did this lab with my Physics students, after their projectile unit test because they did not have to calculate projectiles shot at an angle. It was a way to work in a design challenge with my students while letting them explore angled projectiles.

Students were shown how the shooter works and asked to find the largest horizontal range. They were to record their angle, launch height, etc. and discuss the design changes in between each trial. No additional questions, no conclusions, just a quick and fun experiment about experimental designs.

"Can we stand on the tables Mrs. B?" Sure!
"Can we pull the balloon back all the way?" Sure!
"Can we cut the straw?" Sure!
They just had to record how far it went and how they changed their experimental design.

Seven classes did this lab between my partner teacher and I, usually students worked in partners, spread out across the quad of our campus. We had a running record during the day to see who could in fact make it the farthest. The first few classes hit 38 m, later classes had an unconfirmed 53 m but the largest confirmed was about 45 m. Doing the experiment with so many older students we ran into a few new problems I'd like to warn you about:

Use brand name bullets.

A quick Amazon search brings up lots of refill sets for the small Nerf bullets you need. We opted for a knock-off brand and got 200 bullets for $20. We expected to be set for life as I had previously only broken one Nerf bullet out of 20 with three classes of freshmen testing it last year. We were wrong. Bullets would tear after a single firing, the orange tip would come off upon impact and sometimes even just indentations on the side above the straw was enough to get poor results.



Have extra balloons.
Some of my football players decided to get into a "who can pull the balloon the farthest" contest and frequently broke their balloons. Sometimes it just happened in the course of the experiment. Have lots of extra balloons on hand to repair shooters with duct tape. We tried to use the same size and same thickness balloons for consistency. A few students noticed that the replacement balloon wasn't exactly the same length as before and might change their experiment. 

Careful with metric tapes.
I have one 50 meter windup tape, nine 10 meter windup tapes and one trundle wheel. By the end of the day I had to completely unwind the 50 m one in order to rewind it correctly and we were down two 10 m tapes. Students did not understand how far 10 m was and would run out the tape with such vigor they broke the internal spindle of the tape. They can not be wound again, if you shake them you can hear all the broken plastic pieces rattle around the inside of the case.

The trundle wheel was far superior for measuring and was easier to reset in between trials. Although I did have one student hole the trundle wheel at arm's length straight out parallel to the ground and asked how it worked. He kind of sighted along it, maybe he thought it was a laser level??

Inertia Ball Demo

Many people have some version of this Inertia Ball (available from Sargent Welch, and more) and may use it for an example of inertia similar to how I have in the past. There are several videos online including this one that demonstrate the classic demo (although I don't mention tensile strength yet):

This year I asked students in groups to predict what would happen before I did it. Students were to take a few minutes of discussion; some students came up to inspect the string and gently lift the ball to see heavy it was. I asked each group to share out what they thought would happen when I pulled the string slowly; the majority of the groups correctly guessed that the top string would break. After I did the demo I asked students to discuss again what would happen if the bottom string was pulled quickly. This time groups were split, some saying that the top string would break again and some that the bottom string would break. At this point I introduced students to the word "Inertia," they had not been introduced to it before although several already knew it and start singing the "Bill Nye: The Science Guy" introduction song.

Many of these inertia balls also have a third loop on the side of the ball. I ask students what would happen when I pulled the string from this loop directly to the side. "Are you doing it quickly or slowly?" they ask and I tell them they can think of it either way but when they share out they will have include their choice in their description. Again groups are split, some think that if I pull slowly the top string will break again, others think that if I pull it quickly the side string will break. I pulled the side string slowly at first and students saw the ball shift to the side but the top string held. I briefly said that this looks like a force in the horizontal direction did not affect the vertical direction. I reminded them that we saw a similar directional independence in our projectile unit. I let the ball hang freely again then pull the bottom string quickly and it breaks.

During my first period of the day while students were discussing I decided to add to this demo and make a tennis ball with the similar three eye hooks. I hung the ball and asked the students what they thought would happen if I pulled on the bottom string. Again I let them choose if they would like to think of it being pulled slowly or quickly. This time the top string breaks regardless of the bottom string being pulled quickly or slowly. Without prompting students start discussing why it happens, "Its not heavy enough!" or "See, I told you, it didn't have enough inertia."

While I gave these instructions on my whiteboard I made a powerpoint that has a visual for students as well as the questions.

For high school teachers, this connects well with the NGSS Science & Engineering Practices. Below are the excerpts I thought were most aligned to this activity:

Asking Questions:

O that arise from careful observation of phenomena, or unexpected results, to clarify and/or seek additional information.

O to clarify and refine a model, an explanation, or an engineering problem.

Modeling:

O Develop, revise, and/or use a model based on evidence to illustrate and/or predict the relationships between systems or between components of a system.