Tuesday, December 04, 2018

Epic Rotational Derby

This year in my AP Physics C class I did a Rotational Derby Lab, students have no prior knowledge of rotational inertia formulas and observe races between different objects. The original lab I saw required materials I did not have. I asked some other teachers that do it and got a list together of things I had access to. I ended up with enough of some for eight lab groups and some odds and ends to be shared among them.

All lab groups: cardboard toilet paper roll, PVC pipe, wood dowel of similar radius and lengths; 0.5 kg brass cylindrical mass, hollow plastic ball, ping pong ball, golf ball.

Shared among groups: wood dowels cut the same length and of the same radius as the brass mass, empty oatmeal container, empty pain can, wooden sphere, "sloshy" cans of broth and "nonsloshy" cans of chili.

Students were confused at first at the sheer amount of materials so I made a list on the board for them of everything and added "hollow" or "solid" after each.

"Do we have to write down hollow or solid?" they would ask. I told them they might want to make note of it. Those that didn't ended up adding it later when they realized that information helped them to see patterns in which object "won" the race.

Students were to race two objects at a time down a stack of two whiteboards, releasing them at the same time using a meter stick. Often their predictions were wrong, they usually conducted repeat trials to be sure of their results. There were lots of side experiments going on, racing objects against each other I had not required. (Part of my ulterior motive for having so many objects available to them.)

Their exclamations of frustration or elation would have confused anyone walking by:
"What is with this sloshy can? It always wins!"
"No!!! Tube you were supposed to beat the ping pong ball!"
"Wait .... why the heck did the golfball win?"
"Why did they tie!?"
"No no no do it again, you started the oatmeal before the chili."

I was laughing all day and it was great to see them conducting their own experiments to try and figure out why one won out over another. One student vehemently insists it was voodoo magic.

In one class two groups merged to try and release more materials at once:

Which of course lead to what I wanted to test as well: All the things. At once.

So I grabbed a wooden box I had in the prep room that was wide enough and we set about it. Two students released them all but we found using  two different meter sticks made the release different for the two sides. We used an equivalently long piece of PVC and got the following results:

I took a still from the front video and we ranked the objects as best we could. A grid on this surface could help make it definitive.

1.“Sloshy” can of broth
2.Wood smiley face
3.Golf ball
4.0.5 kg brass cylinder
5.Wood dowels (2)
6.Yellow and red/blue ball
7. Ping pong ball
8. “non sloshy” can of chili
9. Empty paint can
10. PVC Pipe
11. Oatmeal container
12. Cardboard tube

Students had recorded the mass and radius of each object the day they completed the lab. The next day, after learning rotational inertia formulas, they were able to calculated an inertia value for each object. We did not take the different amounts of friction on each object into account but they saw the general trend that the objects with equations that would have higher inertia values if they had the same mass and radius into account were last to the race. The solid objects beat the hollow ones every time; solid objects were #2-5 and hollow cylinders were #9-12. Sloshy vs nonsloshy is a bit more complicated and students don't calculate a rotational inertia value for them. After some information on rotational kinetic energy they realize that the higher rotational inertia an object has, the more energy is going into its rotation and less is going into its translation therefore it will "lose" the race. Applying this idea to the sloshy can students realized that the sloshy contents aren't rotating (much) and therefore that was a lot of mass that was just translating down the ramp. The nonsloshy can however, has a lot of mass to rotate in that can and acts more like a solid cylinder with a lot of rotational energy and less translational kinetic energy than the sloshy can.

The kids really enjoyed the lab and while it was not as accurate as it could be it really helped them to understand what affected an object's rotational inertia and  I think set us up really well for rolling next week. Almost the entire lab fits into three oatmeal containers for storage as well.

Friday, November 16, 2018

You've heard about the new kilogram, but here are the unit changes nobody's talking about

I clearly have no skill in writing clickbait headlines. Anyway...

Let this be our last post about the kilogram. We've had posts on the kilogram



and here.

It seems we have at last moved away from the physical kilogram standard, Le Grand K, near Paris (and it's many replicas around the world). The new kilogram is based on electric current, which seems counter-intuitive at first.

The new definition of the kilogram will change the way we weigh everything

But wait: there's more. The mainstream media's stories omitted changes to the mole, kelvin, and ampere.

Veritasium has the story.

Friday, November 02, 2018

Earth Science Week 5: Astronomy Beyond the Solar System

Okay, the title here might not be the most perfect fit. But it meshes so nicely with the title of Series 4, I went with it.

Stars, exoplanets, galaxies, and such. This is the grooviest stuff in Earth Science if you ask me.

The Hubble retrospective, Nova's Invisible Universe Revealed, puts a catch in my throat and tears on my cheek every time I watch it.

And when you get Alex Filippenko and Neil deGrasse Tyson in the same video (as you do in The Universe's Life and Death of a Star), I'm in!

The hot links to the videos below were active when I posted this, but may well have moved on by the time you read this. Your resilience and resourcefulness will get you access to these documentaries.

Earth Science Series 5: Astronomy Beyond the Solar System

5.1. Stars. The Universe: Life and Death of a Star

The History Channel wants you to go through them to gain access.

5.2. Exoplanets. NOVA: Life Beyond Earth 2. Moons and Beyond

5.3. Hubble Space Telescope. NOVA: Invisible Universe Revealed

5.4. Galaxies The Universe: Alien Galaxies

Here's a link to the Lessons of Phyz question sets at Teachers Pay Teachers:
Earth Science Series 5: Astronomy Beyond the Solar System

Thursday, November 01, 2018

Earth Science Week 4: Astronomy in the Solar System

Having addressed earth, wind, and water in previous sets, it's time to look beyond the atmosphere.

The first time I watched Nova's Meteor Strike, I nearly fell out of my chair. My University of Michigan dorm mate and Physics 401 classmate, Dan Durda, was one of the interviewed scientists.

He and I (and other similarly interested Michigan friends) enjoyed Jim Loudon's Astrofests religiously back in the day. And that 401 class was a shared experience, to be sure. Humbling, but at least it was taught by the outstanding Dr. Jean Krisch, who pointed me in the direction of the inimitable Walt Scheider when it came time for me to do my student teaching.  I was flooded with fond memories when science rockstar, Dan Durda, popped up on the screen.

But I digress. It's a blog; digressions are allowed (if not openly encouraged).

As always, the hot links to videos shown below will likely expire and move on. Use your judgment, resources, and Google-fu to access the content. These videos do tend to be available for purchase.

4.1. Sun Earth Moon. NOVA: Meteor Strike

4.2. The Solar System. NOVA: Life Beyond Earth 1. Are We Alone?

4.3. Stars. NOVA: Secrets of the Sun

Here is the link to the Lessons of Phyz question sets at Teachers Pay Teachers to accompany these videos:

Wednesday, October 31, 2018

Earth Science Week 3: Tectonics, Volcanoes, and Earthquakes

Okay, let's take it down a notch. Right down to the crust. Maybe even deeper.

After yesterday's behemoth, this series turned out to be on the brief side, weighing in at a mere 7 pages of content (7 student pages; 7 key pages). But it's core Earth Science...

Earth Science Series 3: Tectonics, Volcanoes, and Earthquakes

3.1. Plate Tectonics. Naked Science: Colliding Continents

3.2. Volcanoes. NOVA: Deadliest Volcanoes
I streamed this one from the PBS app on my AppleTV.

3.3. Earthquakes. NOVA: Deadliest Earthquakes
I streamed this one from the PBS app on my AppleTV.

Here's the link to the Lessons of Phyz question sets at Teachers Pay Teachers.
Earth Science Series 3: Tectonics, Volcanoes, and Earthquakes

Tuesday, October 30, 2018

Earth Science Week 2: Atmosphere, Weather, and Climate

With the prelims out of the way, this series gets straight into it. The atmosphere, weather, storms, and climate. It's only Tuesday, but this is the most ambitious series of the five I've prepared in Earth Science. Fourteen pages of video question goodness. Focus will be maintained!

The hot linked videos (where available) below may or may not be the best versions that currently stream. Video hot-links are highly ephemeral. Use your Google-fu or purchase hard media versions for better results.

Series 2: The Atmosphere, Weather, and Climate.

2.1. The Atmosphere. National Geographic: Our Atmosphere

2.2. Weather. Modern Marvels: Predicting Weather

2.3. Storms. NOVA: Megastorm Aftermath

2.4. Climate. National Geographic: Before the Flood
This one streamed for free for a short period after it was released. It now streams for a fee. In fairness, it is a major production that is very much worth owning.

Here's the Lessons of Phyz link to the question sets on Teachers Pay Teachers:
Series 2: The Atmosphere, Weather, and Climate.

Monday, October 29, 2018

Earth Science Week 1: Science, Mapping, and Water

I taught a non-lab science Earth Science course a few years ago. I looked for recent-vintage, high-quality documentaries that might complement the textbook resources.

But as mentioned in other posts, I can never bring myself to show a video presentation cold. There needs to be a support document to help keep students focused on the content. The resulting video question sheets have been added to The Lessons of Phyz at Teachers Pay Teachers.

For this post, I'll try to hot link to the videos that go with each question set. Hot links often go cold as time goes by. If a video streams, one's Google-fu doesn't necessarily need to be strong to locate it.

Earth Science Series 1: Science, Mapping, and Water

1.1 About Science. NOVA's The Secrets of the Psychics

1.2 Mapping. NOVA's Lost at Sea: The Search for Longitude

1.3 Water: How Earth Changed History: Water

1.4 Water: National Geographic: Earth Under Water

The question sets on TPT can be found at this link:
Earth Science Series 1: Science, Mapping, and Water

Friday, October 12, 2018

Banked Turns

In AP Physics C students deal with forces in two dimensions, on inclines, acting in circles and more. Inevitably during the unit students will ask the following question in fear:
"Will we ever have to deal with turns on an incline?"

Oh yes kids, but lets build up slowly.

I created this worksheet that referred back to sample and back-of-chapter problems in the textbook. Students worked on increasingly difficult problems with a partner before we reviewed. Here are a key for the worksheet and a key for the textbook problems.

To introduce the concept I delved into the cabinets left to me by my predecessor and found the toys I needed. I raised the whole demo on a wood stand to raise it up. I found a double lane flat track (in yellow) and a banked Hot Wheels track (in orange). Amazingly the Hot Wheels track fit into the inner lane of the flat track. For my first period I used clay and ring stands to hold up straight track to feed cars into each lane. By the next period I added a third straight track to feed one car straight to the wood support, with no turn. By the last period I added color coded signs ...

When I demonstrated this for students I ask students what will happen to the car that goes on the "no turn" track. They correctly guess that it will continue straight so I ask why it doesn't turn. It may take a second but they realized, "There was no force to push it into a circle!"

So then we drop the second car down the "flat turn" track and the initial ramp is high enough that the car rides the edge of the track through the whole turn. I ask students which force kept the car going in a circle and they realize that it isn't the force of friction but the force from the wall in this case. We talked about how that wouldn't be good for "real cars" and that instead we prefer a force of friction to keep our cars going around a turn.

The final car goes down the banked turn and while it may briefly touch the wall it doesn't ride the wall like the flat turn. They notice the banked orange track is about as smooth as the flat yellow track so I wasn't sneaky and just adding friction. Then I asked them what force caused the car to go around a circle and they realize its a portion of the normal force. 

Here is a video of the three cars being sent down the ramps at the same time:

The radius and coefficient of friction for the two turns are not the same and I did not bother to match the masses of the Hot Wheels cars I grabbed. I did not introduce any quantities for students other than identifying which force was causing the motion. It helped for my students to see the differences in the types of turns and they loved being able to run the cars down the tracks themselves. For a quick set-up it did well to help show my students the differences for each type of turn they were going to work through.

Wednesday, September 12, 2018

#AAPTSM18: Alternatives to AP Physics 1 and AP Physics 2

At AAPT's Summer Meeting 2018, I attended session AD: High School, with considerable interest. After a series of College Board-friendly talks by AP Physics Redesign proponents, Mt. Olive High School's Brian Holton presented "AP Physics 1: A Seasoned Perspective".

Holton's talk was clearly not sanctioned by the good people of The College Board. But his expression of frustration and exasperation with AP1 resonated with me. Apparently my expression of frustration and exasperation resonated with him, too. (He cited my lament in his talk.) His critique was much more robust than mine was.

A small group of us had a combination perambulation, ventilation, brainstorm as we migrated to our next sessions.

We concurred that dropping AP Physics 1/2 from a school's curriculum constituted a marketing challenge for any school that would dare to try. We now advertise and market our schools on the basis of the breadth an scope of Advanced Placement offerings and performance.

AP courses are to be added to a school's course catalog; not removed. That other high school being visited by shopping 8th-graders and their parents is offering AP Physics, so your school must match.

One idea we tossed around was running a course that would prepare students for the SAT II Physics exam. Does anyone, anywhere run such a course? I'd love to hear from anyone teaching such a course. For now, it's just a thought. And The College Board still wins.

I know AP Physics C fans are happy with their exams. Abandoning AP1 and AP2 for an SAT II-based course is a different set of conversation. One might argue that outstanding performance on the SAT II Physics wouldn't get students out of any physics course at a college or university. I would hasten to add that outstanding performance on an AP Physics 1 or 2 exam doesn't necessarily exempt a student from intro physics courses at college, either.

Here's what the SAT II Physics exam covers. (A physics content-based assessment: how tantalizing!)

Mechanics 36%-42%
Kinematics, such as velocity, acceleration, motion in one dimension, and motion of projectiles
Dynamics, such as force, Newton’s laws, statics, and friction
Energy and momentum, such as potential and kinetic energy, work, power, impulse, and conservation laws
Circular motion, such as uniform circular motion and centripetal force
Simple harmonic motion, such as mass on a spring and the pendulum
Gravity, such as the law of gravitation, orbits, and Kepler’s laws

Electricity and magnetism 18%–24%
Electric fields, forces, and potentials, such as Coulomb’s law, induced charge, field and potential of groups of point charges, and charged particles in electric fields
Capacitance, such as parallel-plate capacitors and time-varying behavior in charging/ discharging
Circuit elements and DC circuits, such as resistors, light bulbs, series and parallel networks, Ohm’s law, and Joule’s law
Magnetism, such as permanent magnets, fields caused by currents, particles in magnetic fields, Faraday’s law, and Lenz’s law

Waves and optics 15%–19%
General wave properties, such as wave speed, frequency, wavelength, superposition, standing wave diffraction, and Doppler effect
Reflection and refraction, such as Snell’s law and changes in wavelength and speed
Ray optics, such as image formation using pinholes, mirrors, and lenses
Physical optics, such as single-slit diffraction, double-slit interference, polarization, and color

Heat and thermodynamics 6%–11%
Thermal properties, such as temperature, heat transfer, specific and latent heats, and thermal expansions
Laws of thermodynamics, such as first and second laws, internal energy, entropy, and heat engine efficiency

Modern physics 6%–11%
Quantum phenomena, such as photons and photoelectric effect
Atomic, such as the Rutherford and Bohr models, atomic energy levels, and atomic spectra
Nuclear and particle physics, such as radioactivity, nuclear reactions, and fundamental particles
Relativity, such as time dilation, length contraction, and mass-energy equivalence

Miscellaneous 4%–9%
General, such as history of physics and general questions that overlap several major topics
Analytical skills, such as graphical analysis, measurement, and math skills
Contemporary physics, such as astrophysics, superconductivity, and chaos theory

Friday, September 07, 2018

Making Invisible Waves Visible, Near Infrared Imaging - Part 2

With IR Cut Filter                                                                Without IR Cut Filter
In my previous post I showed many uses for a near infrared camera. To be clear, this is NOT a thermal infrared camera.  Thermal IR cameras image in the mid IR and mainly detect electromagnetic radiation due to the random thermal motion of the atoms and molecules in objects. A near IR camera isn't any different than a regular digital camera that creates images from visible light. It is a digital camera that has been modified to make images from the wavelengths that are just beyond the wavelength humans perceive as red. It is like being able to see an additional color that is invisible to the human eye. In this post I will describe how you can modify a digital camera to be a near IR camera.

Remote Control Using iPhone Selfie Camera
The fact that digital cameras are sensitive to IR is a detriment to good photography. The lens of the camera is not designed to focus IR, resulting in a fuzzy picture. The extra IR light can make exposure settings unreliable. To get around this, all digital cameras have a built in IR cut filter, but some IR still gets through. The cheaper the camera, the less effective the IR cut filter. You can demonstrate this by pointing a remote control at any digital camera. When a button on the remote is pressed, the IR LED on the remote can be seen flashing in the camera display. I first discovered this when my 3 year-old daughter was toddling toward me with a remote control as a took a video. I was startled to see it flashing in the viewfinder but not to my eye. She is now 25. The better the IR cut filter, the dimmer the flashing. New phones cut almost all of it, try using the lower quality selfie camera. The IR cut filter must be removed to make a near IR camera. This will void the warranty and possibly wreck the camera. That is why a computer webcam is a good choice for a near IR camera. They are cheap and easier to disassemble.

IR Cut Filter on a Webcam Lens
Webcams have several types of IR cut filters. The worst is a coating on the lens. This must be scratched off, that can degrade image quality. Second worse is a filter attached to the CCD chip. This must be carefully removed to avoid damaging the CCD. The best filters are part of the lens housing. These can usually be popped out without difficulty. The IR cut filters are dichroic, they look transparent straight on but usually pinkish from the side. They are worth saving to explore their characteristics with a spectrometer.

I have converted 5 different webcams over the years. Because I like to use a Mac, it was more difficult do find suitable webcams. Macs have come with built-in webcams for a long time, so few vendors make Mac compatible Webcams. The most recent near IR webcam I converted is the Logitech HD Laptop Webcam C615. It can be purchased for about $25. It was a little difficult to take apart until I found out I could peel off the flat plastic panels on the front to reveal the screws. The IR cut filter was glued on to the lens housing but easily broke off with a quick blow from a pencil. It worked well with my Macbook Pro and is my only HD near IR webcam. I made a video showing the conversion process for this webcam in case you want to give it a try.
My other converted webcams are older models but still available as of this writing. I tried the IceCam2 by Macally. It worked well with my Mac but unfortunately the IR cut filter was a lens coating. I was able to scratch it off with an X-Acto knife with acceptable results, but I wouldn’t recommend it. The Macally MegaCam had a removable IR cut filter and works well. The only downside is it had a limited range of focus. Next I tried a Logitech QuickCam Connect. Although there was a Mac driver when I made it, currently there isn't one. The IR cut filter was a small square piece of plastic that easily popped out of the housing. This camera worked well on a PC and my Mac laptop running Windows. I recommend this one for PC users. I then tried the Logitech QuickCam Chat Web Camera because there was a Mac driver available for it. Unfortunately it had an IR cut filter coated on the lens. I swapped the QuickCam Connect lens with this webcam. This is what I used until a Mac OS update caused that driver to stop working too!

Once the IR cut filter is removed, replace it with a filter that only passes IR radiation. I use a Wratten #87C filter. Edmund Optics sells one for $175. That is expensive but it would be enough to make many near IR cameras. The filter is large enough to be attached over the lens of the camera. This allows pictures to be taken with and without it for comparison. I chose to place it inside the lens housing of the webcam so it is more secure. I can then use an unmodified webcam for comparison pictures. You can find less expensive Wratten 87C filters on eBay for $30-$40.

Unexposed, Developed Color Film
A much less expensive filter can be made from developed, unexposed color film. The resulting dark negative works almost as well as a Wratten 87C, see spectra below. If you look through old boxes of color negatives at your grandmother's house you will probably find some developed, unexposed sections on the ends of negatives that you can use. They are the portions that look completely dark. This material will pass IR while blocking almost all visible. Some people recommend floppy disk material but it passes a lot of the visible spectrum. I don't recommend it for near IR webcams. However, looking directly through it gives you an approximate view of what things look like in near IR.
View of My Classroom Through the Material from a 3.5" Floppy Disk
The figure below shows the continuous spectrum of an incandescent light (red line). The other curves show this spectrum after passing through various filters. The sensitivity of the spectrometer was adjusted to show the spectra at approximately the same scale so the intensity values can't be compared. The purple line is the IR cut filter that was removed from a webcam. You can see why these must be removed as most light above the 650 nm wavelength is blocked. The orange curve is the material from a 3.5" floppy disk. Although it passes IR, it also allows a lot of red and orange light to pass too. The green curve is developed, unexposed color film. It works almost as well as the Wratten 87C (blue line), letting through only a small amount of visible light. If you use developed, unexposed film as a filter, try doubling it up for better results.
Continuous Spectrum Compared to Spectrum After Pass Through IR Cut, Floppy, Color Negative, and Wratten 87C Filters
You can use any software that displays a video preview of your webcam to capture images. On a Mac this is Photobooth if you have a compatible webcam. For PCs, the software that comes with the Logitech webcams works well. It has settings for low light levels, still and video capture, and some basic editing tools. There are many other choices for PC webcam software including ManyCam

A near IR webcam is very useful in the classroom but awkward for taking around town and country. Many people would have difficulty successfully taking apart a webcam AND putting it back together in working condition. I know I did. Another option that overcomes these obstacles is the Sony Nightshot line of cameras. They have a switch that slides the IR cut filter out of the optical path. They also have IR LEDs to illuminate objects so they can be seen in total darkness. I use a Sony MiniDV Handycam DCRHC40 that I purchased new in 2004. There are usually many inexpensive Sony Nightshot cameras for sale on eBay. To convert a Nightshot camera to a near IR camera all you need to do is place an IR pass filter over the lens. Either a Wratten 87C or one made from developed, unexposed color film works well. I made a card stock holder so I can quickly attach and remove the filter to take near IR/visible comparison pictures. I also cover the IR LEDs with electrical tape although they are useful for some applications.
My Sony Nightshot Camera With IR Pass Filter Installed and Tape Blocking IR LEDs
Sony Nightshot cameras have been manufactured since 1998. There was some media attention when people used them to see through certain fabrics when illuminated with a bright near IR source. Sony modified them so that they overexpose in bright conditions to prevent voyeurism. This does not affect their use as near IR cameras because the IR pass filter dims the image considerably. However, I sometimes have issues with overexposure in bright sunlight. There is a way to defeat this, set the Nightshot switch halfway between on and off. That is how I took the picture of the billiard balls at the top of this post.
Fabric in Visible Light                                                                  Same Fabric in Near IR
Another option would be to modify a digital camera like a webcam by removing the IR cut filter and replacing it with an IR pass filter. This is best done with an old digital camera that would not be missed if it refused to work after reassembling it. I tried this with an old Kodak digital camera and it did work for a little bit, then became e-waste. There are companies that will do this but their main purpose is to remove IR cut filters from DSLR cameras to make them better for astrophotography. If you are interested in pursuing this option, here is a clearinghouse of information and here is a good place to start.

I learned how to convert webcams to near-IR cameras by searching for DIY websites and videos online. This post is an amalgamation of what I have learned from this research and from using near IR cameras for many years. Here are a few websites that I found useful: