#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!

Egg Toss video because—why not?

The final batch of clips from Grass Omelette XVII at Rio Americano. A nice catch and two spectacular splashes! With no sun, we were able to line up with the geometry of the field. Seems we should have taken a bit of Sharpie to the eggs to increase their contrast with the sky.

Large impact time to reduce impact force


Impact force too large #1



Impact force too large #2


Previous Grass Omelette XVII coverage can be found here:
Splash and Catch: Grass Omelette XVII

More robust coverage of this activity can be found here:
Egg Toss 2013

Solar Tile Impact

Much of the tech world is abuzz with Elon Musk's announcement that Tesla will be producing glass solar roof tiles. They look as good as normal tile or slate or other roof materials but they are inconspicuous solar tiles. There are tons of articles out there but this video posted to @TeslaMotors twitter page caught my eye:

Solar roof glass tile vs. conventional roof tile pic.twitter.com/AnGWJ07jub

— Tesla (@TeslaMotors) October 29, 2016

A kettlebell (I assume at least 10 lbs) is shown falling directly onto each tile sample being dropped from the same height and edited to be at the same time. The different materials respond differently; while the solar cell may not be functional after such a hit it structurally remains in one piece unlike the others.  But I was caught by the varying rebound heights. I downloaded the video and opened it in Vernier's Video Physics app on my iPad and started playing with it. This is the first time I had used it to follow an object's entire motion so its not the cleanest. I tracked the first kettlebell that fell onto the Terra Cotta tile on the far left:

I plan to use this in my energy unit. I can ask students to discuss the change in potential energy for each of the kettle bells as they fall. Students could look at the rebound height for each sample and discuss the loss of energy in each case. I'm hoping students will realize that the loss in potential energy means that the energy has gone elsewhere. You could discuss common product testing, brittle materials vs elastic ones, momentum and more. If you have one-to-one devices you could have students do the same analysis for each material so that they can get the same information for each.

This time machine makes a second last for twelve minutes

And it's only $2500 for the privilege.

Chronos 1.4 is the brainchild of engineer David Kronstein, who first demonstrated the camera’s hardware and recording capabilities with a production-level prototype under his YouTube handle ‘tesla500.’ The camera can record 1,057 fps at 1280 x 1024, and up to 21,650 fps at lower resolutions.

The math pencils out as follows:

When viewed at the standard video frame rate of 30 frames per second (fps), 21,650 frames would take about 722 seconds, or 12 minutes to watch.

The iPhone can natively capture 240 fps, which slows 1 second of action into (240/30) 8 seconds of viewing.

DPReview has an article on the Chronos 1.4, and there's a demo video as well.

Chronos High Speed Sneak Peek



UPDATE: Watching the sneak peek makes me think the native playback rate on this camera's files is 60 fps, in which case 1 second of action will blow by in a mere 6 minutes at the highest frame rate.

UPDATE 11/25/16: The Kickstarter campaign goal was achieved in five hours.

Using a laser to pop balloons—with a surprise

AAPT's SM16 included the customary picnic and demo show on its last night (which precedes its last day).

Rutgers' David Maiullo, star of That PHYSICS Show led a capable crew of physics demo artists through a fun-filled demo show.

I caught most of the "balloons popped by a laser" sequence (at 240 fps). But upon further review, I noticed something unexpected. Who doesn't love something unexpected (in this context, anyway)?

Have a look. What surprised me, and what's the explanation?

Laser Balloon Pops

Shattering Pyrex at 344,000 frames per second

I attended a high-speed video workshop at the AAPT Summer Meeting 2016 in Sacramento. It exposed me to Tracker, which I had not used before.

I've long been a fan of high-speed video, as you can see if click the "high-speed video" label off to the right in the labels section.

While intending to scope out the newly-prized Sony RX10 at my favorite camera gear website, DPReview, I serendipped into a post noting that The Slow Mo Guys had posted footage of shattering Pyrex at various recording rates.

They cranked their camera (a Phantom v2511) up to 11 by dropping the resolution. This allowed them to capture at a ridiculous rate of 343,900 fps. As is their wont, further shenanigans ensued.

Glass Explosion at 343,000FPS! - The Slow Mo Guys

Colliding Bees

Recently we hosted three honey bee hives on our property to help a local beekeeper. We loved seeing more of the little guys in our garden and it was fascinating to watch them. While they can zip and zoom around so fast its amazing to watch them seemingly hold still when they hover a bit. Well its even cooler in slow motion:

Its just funny watching these graceful flyers bump into each other. But there's Physics going on too! The midair collisions demonstrate a few different principles; you can show the video to your students and ask them to identify some.

• Newton's Third Law: Often as the bees crash into each other they rebound back in the opposite direction. When they don't, what is different?
• Conservation of Momentum: The bees are approximately the same size and do not become entangled they can bounce apart. What do you know about the the total momentum of the system (two bees) before and after the collision? For higher level students, could you analyze the video in 3D?
• Ask students what else they notice while watching!

Ramirez, you broke Physics

Not really. Although when you only get a narrow view like the video below of Cleveland Indian's Jose Ramirez it can seem like it.

A wider view (barely) shows the kick that sent the helmet flying up and forward making it a projectile moving horizontally the same way Ramirez was although probably not at the same speed.

WIRED Magazine has a nice write-up, with the physics behind it to boot. The article shares a quick projectile demo that many of us use, illustrated by these videos as well:

Vertical Projectile Launch from a Horizontally Moving Cart:

Walter Lewin's "Horizontal Motion remains constant" from MIT:

So how can you use this in the classroom? I use this demonstration to illustrate the fact that the horizontal motion and vertical motion of a projectile are independent from each other. You can tell students this over and over and over again but for some it is one of those conceptual sticking points and they just don't understand.

When I demonstrate this with a ballistic cart in my classroom I first give the cart that will launch the ball straight up a small push so it moves slowly. The second time (because they will all want to see it again) I give it a big of a bigger push. If you want to have a faster horizontal velocity you'll want to really make sure your track is level and the cart doesn't rattle on the track. If the ball doesn't land back in the cart, usually through a funnel type mechanism as seen above, your students won't believe you. So take care, their comprehension depends on it!

Even after students have seen this demonstration I think the video of Ramirez is something to share. I'm going to start with the narrow point of view and ask them, "What happened?!" with no other information. [Depending on your level of students feel free to add an expletive from dramatic affect.] Just based on their previous knowledge of projectile motion, and hopefully due to careful observation, they should realize the helmet has some initial horizontal motion before it separates from Ramirez. Ideally, a discussion follows when someone realizes, "Wait, he totally kicked it!" and students can then focus on what that kick (ahem, force) did to that helmet.

As Dean Baird says, sometimes we have to pretend to be the dumbest person in our classroom so that we can lead students to their own understanding through critical thinking. I'll keep asking them questions, usually with a bit of "How do you know that? What would that mean? What does that remind you of [that we have learned about]?" Once students have gotten close enough to the "real reason" this happens I plan to show them the wider angle and perhaps printed copies of the WIRED article to solidify the whole experience.

And if you want a funny ending to it all, the Cleveland Indian's mascot, Slider, tried to help Ramirez out:

Serendipity with Center of Mass Demo

Recently I set up a demonstration that I hadn’t done for a while. You balance a dynamic track on a thin board. You then place two carts with different masses on it so the center of mass of the two-cart system is directly over the thin board that acts like a fulcrum. The track stays balanced. I used to tie the carts close together with a thread. Their magnets would repel and send the carts flying apart when I burned the thread with a match. This was a little tricky to set up and difficult to repeat quickly if something went wrong. I decided to try the spring-loaded plungers built into the carts. I avoided this before because it requires touching the carts to release the plunger.

It turns out this wasn’t a problem, the carts still separated cleanly. This made doing the demo a lot easier. I was in my room on the weekend trying this out and decided to make a video of the demo in slow-motion. This would be good for discussion and as a back-up if things went wrong. I marked the initial position of the center of mass of the carts with a white sticker. The blue cart is 0.75 kg and the red cart is 0.25 kg. Here is the result in 300 frames per second:

After viewing the video I was thinking about how it showed that the internal force of the plunger did not affect the velocity of the center of mass. It was zero before the separation and zero up to the point an external force acts on the red cart hitting the end of the track.

I thought if I could get the carts to move back toward each other with the same speed, the track would stay balanced too. This would be difficult to arrange in reality, but with video editing, it was easy. I imported the video into iMovie and used the rewind feature. Now it shows the two carts heading toward each other, compressing the spring, then flying back out. The track stays balanced the whole time, even with the excited physics teacher flapping his arms around and poking the blue cart with a pencil during the collisions, see for yourself:

This is what every one-dimensional elastic collision looks like if the velocity of the center of mass is zero from your frame of reference. There is a well-known technique exploiting this fact that makes solving for the 2 final velocities of an elastic collision much easier algebraically than using conservation of momentum and energy. I have shown this technique to my students before but felt they really didn’t understand why it worked.

This video seemed to be a good tool to try and improve their understanding. They see that if you are in the center of mass' frame, in an elastic collision the carts change direction after the collision, retaining their original speed. All you need to do is first subtract the velocity of the center of mass from their initial velocities to put yourself in the frame of reference where vcm is zero.

After switching the direction of the speeds, you need to go back to the rest frame by adding it back to the final velocities. The video gives the algorithm for doing this more meaning for the students:

1. Find vcm, it equals the total momentum divided by the total mass.
2. Subtract vcm from the initial velocities.
3. Change the sign of the result. You now have the final velocities in the center of mass' frame of reference.
4. Add vcm to the final velocities, you now have the final final velocities in the rest frame of reference.

I suggest having students use video analysis to implement this technique. I still want my students to be able to write out the equations for conservation of momentum and energy. They can do it after using the center of mass technique to check their results.

Ice Scream - a new ExploratoRio Science Snack

When broadcast television was something people watched, networks would pitch their "encore presentations with the line, "If you haven't seen it before, it's new to you!"

Somewhere in the running conveyor belt that is my Facebook feed, I recall seeing a video of a Kennedy half dollar coin being set into a cavity in a chunk of dry ice. The coin and ice produced vibrations and sound.

When two students ran me through the paces of their Bubble Suspension build (for our upcoming ExploratoRio2016 on Wednesday, April 13), they left some dry ice behind in my classroom sink.

So it's me, some sublimating dry ice in a sink, and no one else around. Recipe for serendipity if ever there was one.

After dousing the frozen carbon dioxide and blowing the subsequent cloud out of my sink a few times (that never gets old), I found a nearby electrophorus and pressed it into the sublimating mound.

What happened next is best relayed in the form of a video, which drops the action to 1/8th speed at about 20 seconds in.



So unexpected was the sound that I decided a new ExploratoRio Science Snack was in order. The title of "Ice Scream" wrote itself with little help from me.

The details (as much as I know them or foresee them) are in the recipe linked to below.

Ice Scream - ExploratoRio Science Snack.

Note: Adopting the look and feel of the original Exploratorium Science Snackbookq was not an accident.

Van de Graaff Ping Pong

As played with a bundle of silk thread, as you do.

This gem comes to The Blog of Phyz from Inderkum's Jessica Downing, an upbeat, resourceful teacher at the most amazing high school in America.

It's a nice sequence of polarization, charging, repulsion, and discharging—rinse and repeat. It's like a puppy playing fetch.

This video below is slow-motion. This link will take you to the normal-speed version.

Van de Graaff Demo

Groovy… but I won't show it in class

Except as a springboard to a discussion of "What did they do wrong this time?"

But it is groovy. The world's largest vacuum chamber is used to perform the a variation of age-old physics classic, "penny and feather" free fall experiment.

Brian Cox visits the world's biggest vacuum chamber - Human Universe: Episode 4 Preview - BBC Two



Here's a video clip that I do show in class: A hammer and a feather dropped on the Moon.

Feather & Hammer Drop on Moon



Brian Cox is many kinds of wonderful, but showing free fall in a vacuum chamber using high-speed (slow motion) video, alone, acts to deceive.

A common misconception among physics learners is that gravitational acceleration depends on atmospheric pressure. Things float in space because there's no air in space. There's no reason to think g in the giant vacuum chamber is 9.8 m/s². All video of free fall in the evacuated chamber is artificially slowed. We never see the vacuum chamber free fall in real time.

The lesson could be interpreted that things fall more slowly in a vacuum. On Earth as it is in Heaven. Or the Moon, at least.

AAPT SM14: Physics Force selected moments in high speed

The Physics Force provided the demo show for AAPT's Summer Meeting 2014 at the University of Minnesota in Minneapolis. I caught a few moments at 120 fps in HD and posted the compilation to YouTube.

Physics Force AAPT SM14

Some high-speed video from NSTA 2014 Boston

The 2014 NSTA conference was held last week in Boston. A few clips of high-speed video were made available to us, so we present them here.

Frisbee Dogs
There was a session on using Frisbee-catching dogs in the physics curriculum. Some ask "Why?" The answer is that there are people out there who love physics and love dogs, so they brought their passions together. In any case, they also brought a dog to the Boston Event and Convention Center.

Frisbee Dog (HD at 120 fps)

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Pasco's Annual "Just Physics" NSTA Demo Show
The good people at Pasco once again staged their "Just Physics" demonstration show. David Maiullo assembled his blue-ribbon crew: Sam Sampere, David Sturm and Borislaw Bilash II. "Just Physics" always draws a big crowd, and serves as a welcome refuge for any physics teacher overwhelmed with all the non-physics going on at the conference. Pasco provides food, T-shirts, and resources for a first-rate show.

There were too many demos in the 90-minute show for anyone to capture them all. And not all of them lent themselves to high-speed video.

Ping Pong Bazooka
We'll begin with a clip that reveals challenges to the demonstrators and to the videographer. The Ping pong bazooka is now well-known among high-end physics demonstration professionals. But it can be finicky, and it doesn't always perform well under, uhm, pressure. The camera being used is a Panasonic Lumix DMC FZ200. It's capable of high-speed capture at 240 fps (VGA) and 120 fps (HD). The camera also sports a telezoom lens whose focal length can be extended to the equivalent of a 600 mm lens with an aperture of f/2.8. While the lens features image stabilization, going to full zoom challenges the IS system.

Ping Pong Bazooka (VGA at 240 fps)

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Ping Pong Explosion
We move on to the Ping Pong Explosion. A plastic waste can has a bit of water at its bottom. A small, empty, plastic soda bottle is partially filled with some liquid nitrogen then capped tight. The bottle is tossed into the can and immediately covered in hundreds of Ping Pong balls. This part needs to be executed with some haste. The liquid nitrogen boils as the liquid turns to gas. But the gas requires much more volume. And that volume is not available in the small soda bottle. So this happens:

Ping Pong Explosion (VGA at 240 fps)

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Liquid Nitrogen Cloudburst
In out final high-speed clip, we see what happens when a cooler of liquid nitrogen is doused with a bucket of water.

Just Add Water (VGA at 120 fps)

Lightning shot at 11,000 FPS

PetaPixel has an article about some incredible high-speed lightning video. And they link to the goods!

Incredible High Speed Video of Lightning Captured at 11,000 Frames Per Second

Here's the clip posted to YouTube.

Incredible Slow Motion Lightning Strike! (1 sec. = 3 min!)



I had never seen the "sparkling" high in the clouds as the stroke develops. So much randomness before a channel is established. The things you can see with high-speed video!

Egg Toss 2013

Challenge: A penetrating puzzle that could spin your mind

Apparently today's theme is "Shooting bullets into things... for science!"

Take a look at this mechanics puzzle, presented in a YouTube video. You'll need to render a prediction to see the outcome.



The solution? You're on your own for that.

Hat tip: Laurie Miller Tarr

20 kg of metal at Mach 1 meets 10 Mg of ice

And hilarity physics ensues.

The European Space Agency slammed a jet-pack accelerated, 20-kg "penetrator" into a 10-metric tone ice cube.

They had their reasons.

The impact decelerated the penetrator to the tune of 24,000 gs. Some folks pass out if exposed to 10 gs, and the human body loses structural integrity around 30 gs (300 m/s^2).



In addition to the obvious grooviness, I see introductory physics problems for homework and tests. You?

Hat tip: Phil Plait's Bad Astronomy

Bead chain dynamics

Fun stuff from Earth Unplugged via NPR's Science Friday's Facebook page.

Amazing bead chain experiment in slow motion - Slo Mo #19 - Earth Unplugged


I'm not sure what meaningful high school physics pedagogy is here. For now it's mainly "gee whiz" cool.

Not that there's anything wrong with that.

UPDATE: Bead chain seems to be more commonly referred to as ball chain. If you look for it, it will be listed with a size. The sizing regimen is one of direct proportionalities (unlike wire gauge). Here's a handy guide page: Ball Chain Sizes. The chain in the video looks to be about size 10, but that's a mere guess on my part. And I'm not always the best guesser.

UPDATE 2: Further research shows my size guess was correct. More ball chain goodness can be found at The Kids Should See This. Empirical Zeal's robust analysis merits a link!

The Newtonian Shot in high-speed

Two dart guns are fired straight downward at the same time. But the darts have been modified. One dart has a cork ball attached. The other has a steel ball attached. Which one hits first?

In class, we engage in a guided think-pair-share discussion. The guiding document can be found here:

The Newtonian Shot

I generally get good arguments for the light dart hitting first and for a tie. I'm lucky to have such engaged students!

Only after all arguments have been expressed and represented do I proceed with the demo. In class, I shoot the darts from guns suspended from a support bar structure attached to a ceiling I-beem.

For high-speed video goodness, I shot the pair outside in the blazing late afternoon light in sunny Sacramento.

Here's the 600-fps clip. Enjoy that cork ball bounce!



Here's the 1200-fps clip.



You might reasonable worry about finding the type of spring-loaded dart gun that works for this demo. You need a hard stick dart—Nerf darts or soft rubber darts simply won't do. They seemed to be available everywhere when I was a kid. But safety worries and lawsuits made them an endangered species. Fortunately Arbor Scientific is able to get them.

Arbor Scientific Dart Gun