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:

Wednesday, August 29, 2018

Making Invisible Waves Visible, Near Infrared Imaging - Part 1

Simone models her scary giraffe costume in visible and near IR
The human eye is not capable of detecting wavelengths much larger than 700 nm. However, digital cameras have the capability to detect near infrared radiation a little beyond 1000 nm in wavelength.  This is almost as large a range in wavelength as the entire visible spectrum! Digital cameras are able to detect this electromagnetic radiation and reveal it as images and movies. This allows many creative and novel ways to explore IR radiation in the classroom and beyond. For example, clothing looks different in near IR as the above Halloween costume reveals. In this post I will give examples of near IR images I have made. In Part 2 I will describe how to make your own inexpensive near IR camera.

There are many things that can be explored with a near IR camera. One of the more surprising things to observe is paper money. The back of $10, $20, and $100 bills have strips of near IR reflecting pigment that obscure the printing behind it.
The reverse side of a $20 bill in visible and near IR
I remember showing this to a laser physicist and he got so excited he went home and made his own near IR camera, and he worked every day at the National Ignition Facility at LLNL! The patterns are different allowing scanners to quickly and accurately differentiate between denominations as these images of other denominations show.
The reverse side of a $10 and $100 bill in near IR, notice almost invisible 100 on right
The counterfeiting measures on the front of the $100 bill look very different in near IR. The inkwell disappears revealing the Liberty Bell. This happens in visible if you tilt the bill. The fountain pen and phrases from the Declaration of Independence disappear and the security ribbon becomes very faint.
Obverse of $100 bill in visible and near IR
It is interesting to watch appliances like hotplates, toasters, hairdryers, and a range top burners warming up. These are all easily visible in near IR even when there is no red glow yet detectable to the eye. The range of brightness is much greater than seen in visible. Below is video of me touching a hotplate when it was just warm but glowing in near IR.
Near IR passes through black plastic bags. You can shine an IR LED through it or place IR emitters like this space heater behind it and see right through the bag.
Visible and near IR image of a black plastic bag draped in from of space heater
Fluorescent light sources are dimly visible, the warmer ends showing up brighter. Compact fluorescent lights show up brighter than standard fluorescent tubes. Reflected fluorescent light is mostly invisible. A room illuminated by only fluorescent light is dark.
My classroom with all the fluorescent lights on in visible and near IR on a foggy morning
Incandescent lights are very bright and can light up the whole room with near IR. The filament can be seen for a few seconds after they have been turned off. Different color LEDs are interesting. Red shows up brighter than other colors. The red LED has a peak wavelength in the visible but still emits light on either side of this peak, some in the near IR. Most of the near IR from the red and other LEDs is probably from thermal radiation. IR LEDs show up as intense lights. Remote controls have near IR LEDs and can be used as near IR flashlights to “see” objects in an otherwise dark room.
The red LEDs of my binary clock and shining a remote control on my face in the dark with a remote control shining on it
Smoke and dust particles scatter visible light better than near IR. As a result, near IR reveals distant objects that are obscured in visible light.
South San Fransisco Bay Area from Saratoga, distant mountains only seen in near IR include Mission Peak
There was a small wildfire in North Cascades National Park during our trip in July of 2018. I took several photos of the mountains in visible and near IR. This shot of Colonial Peak turned out the best. Notice the extra details visible in near IR and the smoke is less of a problem. The shadows are more stark, showing that near IR does not scatter as much as visible. Trees reflect near IR more than visible, showing up brighter.
Colonial Peak looms over Diablo Lake in North Cascades National Park
The Los Gatos High School pool looked very different in IR. Very little IR is coming from the bottom of the pool so the lane markings on the bottom can't be seen. The sky reflected from the surface looks dark making it look like a senior prank involving ink has been perpetrated. Other reflections off the pool are more visible like the flags and light poles in near IR.
Los Gatos High School pool in visible and near IR
There are many other things to check out, I won't show them all here. LCD monitors show up as a dull glow with nothing showing from the display. Some have a bright corner where the back light is located. Red lasers are absolutely dark unless you shine it directly into the camera. The resulting dim glow is probably thermal radiation. Different items of black clothing that looks identical in visible can have dramatically different shades in near IR. Plants tend to reflect a lot of near IR but this varies by species.

I hope I have piqued your interest in near IR imaging. Near IR cameras can be made by modifying a webcam. All you need to do is replace the IR cut filter with an IR pass filter. Another option is to purchase a Sony Nightshot camera and place a near IR pass filter over the lens. You can find used Nightshot cameras on eBay for less than $40. I will describe both of these options in detail in my next post. If you can't wait, there is a lot already posted on the Internet, search "DIY infrared webcam".

Friday, August 10, 2018

#AAPTSM18: Garbage can smoke rings

I will post as many gems as I can from the American Association of Physics Teachers Summer Meeting held in Washington, DC July 28-August 1, 2018. On Twitter, that's #AAPTSM18. The items may be new or classic; simple or complex. Here's one such gem.

Dave Maiullo shows us his smoke ring cannon, adapted to the constraints of the room. Like most things, ring vortices look even better in slow motion.

Maiullo Smoke Rings

Bonus: Maiullo Dragon Voice!

Tuesday, August 07, 2018

#AAPTSM18: Using physics to win a tug-of-war

I will post as many gems as I can from the American Association of Physics Teachers Summer Meeting held in Washington, DC July 28-August 1, 2018. On Twitter, that's #AAPTSM18. The items may be new or classic; simple or complex. Here's one such gem.

At the "30 Demos in 60 Minutes" session, Colorado State's Brian Jones—director of The Little Shop of Physics and author of College Physics by Knight, Jones, and Field—showed us his secret to winning a tug-of-war.

Brian Jones Tug-of-War

I love it: so simple!. Two concerns:

Those once-ubiquitous single-use plastic bags may be an endangered species as voters outlaw them out of concern for the growing plastic islands and gyres in the oceans. It should work equally well with the thicker multiple-use bags that are now coming online.

I wonder if this will work as well on tile as it does on carpet. It clearly works like a charm on carpet. I guess I'll find out soon enough how well it works on tile.

Sunday, August 05, 2018

#AAPTSM18: Google Science Journal

I will post as many gems as I can from the American Association of Physics Teachers Summer Meeting held in Washington, DC July 28-August 1, 2018. On Twitter, that's #AAPTSM18. The items may be new or classic; simple or complex. Here's one such gem.

It is not rare when I spend time with my colleagues at AAPT meetings that I feel like I missed a memo somewhere along the line. If my head is swelling with overconfidence, mixing with my very talented, informed, creative, and knowledgeable colleagues puts me back in my place.

Such was the case when someone casually mentioned the use of Google's Science Journal during AAPTSM18. What? I have a bunch of Google apps, but I hadn't heard of Science Journal. Why not? I don't know. Sometimes I'm just not paying attention.

Anyway, Science Journal accesses sensors built into phones (or as we older folks think of them: smart phones or cell phones) and manages the data for easy use by phone users, such as students. Take a look.

Science Journal

Science Journal, an initiative by Google #ScienceJournal

I don't allow phone use in my class. Strictly and absolutely. Unless I invite exceptions. Phones are never far from students' hands. Given a green light, the phones can be out and about in no time. I'm not sure if I'll find a use for this in class. But at least I know it's there. Hmmm... experiment-based homework?

#AAPTSM18: Color mixing with Ikea's Ledberg

I will post as many gems as I can from the American Association of Physics Teachers Summer Meeting held in Washington, DC July 28-August 1, 2018. On Twitter, that's #AAPTSM18. The items may be new or classic; simple or complex. Here's one such gem.

Rutgers University demonstration specialist, Demo a Day co-author, and That Physics Show star, Dave Maiullo, presented many demonstrations, as he does. With trademark Maiullo panache, of course. One of them was the use of Ikea's Ledberg color-changing LED light strip in conjunction with diffraction ("rainbow") glasses. The LEDs in the strip act as nice point-like sources, a controller allows for variation of the emitted color, and the diffraction gratings produce a nice spectrum that changes in correspondence with the LEDs.

Here's Dave's presentation to a group of physics teachers (Session CM: 30 Demos in 60 Minutes). [Note: this is operationally a family affair as many folks in the room know each other and the demonstrators; so the atmosphere is familiar and casual.] As an attendee, I did my best to capture as much of the session as I could. I did better with some demos than I did with others. My rainbow glasses were secured beyond my ability to deploy them while recording the demo. That didn't stop me from trying!

I managed to make an effort to hold the glasses in front of my big camera during the official demo show. A bit slapdash; I'm delighted it worked as well as it did to show the spectra of the LEDs off to the right.

Tuesday, July 31, 2018

Physics teacher conferees treated to a demo show par excellence

Images from the 2018 Summer Meeting Demo Show, July 31 at the Renaissance Hotel & Conference Center in Washington, DC. Show produced by AAPT's PIRA Team (ft. David Maiullo and Stan Micklavzina) in conjunction with the Bubble Magic of Tom Noddy.

2018 07 AAPTSM18 Demo Show
2018 07 AAPTSM18 Demo Show

Thursday, July 26, 2018

Vintage transformer

I've seen a meme going around recently that says,

"It's not hoarding if your sh!t is cool."

True. And this is the excuse I will give for keeping old vintage equipment that still demonstrates some physics. The same could be said for my father-in-law, a retired hospital mechanic that got to keep a lot of old equipment when it was replaced. He recently gave me a National Twin-Control Transformer, pictured below. It was used in the surgical wards to change the voltage of the lights (left dial) and cauterizing gun (right) from the 120 V wall outlet. I found a few more of these online for sale as interest pieces, luckily no one suggested actually plugging it in. There is an on/off dial at the top and what appear to be banana-type lead ports underneath each control.

The art deco style seem to be from the 1920s or early 1930s. The plastic shell is Bakelite, the first synthetic plastic for which the American Chemical Society named it a National Historic Chemical Landmark. One of its most common applications were casings like this for electric devices because it was very electrically resistant.

I took the front of the case off so that I could see the very large transformers inside. My father-in-law had cut the cord internally, just in case any students decided to get too curious. The transformer wire gauge is quite large and the whole mechanism quite heavy. I took it out to show some students but I'm concerned about some of the wrap components so it will probably be left sealed up in between demos.

It would be interesting to encase the transformer opened up so students could see it but it was safe from curious hands. Its a reminder that what we teach them is actually used for something and hopefully they are able to recognize similar components.