Conservation of Momentum in Explosions

Yesterday I did the conservation of momentum demo with "energy sticks" in some parts of the country they use fire crackers, but those are illegal in Michigan.* The basic set up is a short section of 3" diameter PVC pipe with a small hole drilled through one side. You put a pop can in each side and put an energy stick in the hole. The explosion propels the cans out the ends of the PVC pipe.

Both cans will have the same momentum just in opposite directions, so that total momentum is zero. You can change the mass by adding some sand to one or both cans and see how this effects velocity. In order to get velocity all you need to do is solve a simple projectile motion problem. Or you can do video analysis, but your milage will vary here. The can leave the PVC at anywhere from 3 m/s to 9 m/s or so.

I forgot my pop cans yesterday so I ended up finding some Arizona Ice Tea cans. These worked pretty well (mass = 25 g plus or minus 0.5 g). Then I used a Chef Boyrdee can I had laying around as an unknown mass (ended up at 48 g). My students were tasked with finding the mass of the ravioli can. The answer they arrived at was 36 g instead of the 48 I got from a balance. My guess is the inaccuracy comes from the deformation of the ice tea can, differing geometries of the bottoms, and the slightly different diameters of the two cans. But a good problem to work anyway.

*I'm not sure if it's illegal to use them or if it's only illegal to buy them, or how the legality goes in an educational setting.
**Link to the zip file of videos pending.

One Reason to Give Presentations

I really enjoy giving presentations to other educators, it's probably related to why I love being a teacher. The major difference is that when I present to my colleagues they generally want to learn what I'm trying to teach and this makes it even more fun. This is not why I'm writing today. I'm writing today because I just had an epiphany. I discovered another reason to love giving presentations.

My new thought came while preparation to lead a workshop in the use of video analysis in teaching. It's going to be a six hour long workshop.* Originally I was mainly going to focus on the how, but with six hours I have more than enough time to present the why as well. It was while getting these thoughts in order that the light bulb went on.

I have to think of well reasoned justifiable reasons to present to other physics teachers to convince them to try what I'm teaching. Do I have them? Is there research to support them? Or is it all just fluff and hand waving meant to grab attention. I have reasons, but I'm not an educational researcher, but physics education is one area where there is lots of research, so I've been told. So I went to the internet and my bookshelf.** It turns out, that according to research, my gut feelings were right. I'm not totally surprised by this as I go to lots of meetings and talk with lots of people. All of these interactions added to my own classroom experience have led to my "gut feelings".

Anyway, I'm not writing to say, "Woo, Hoo I was Right!" I'm writing because the process of researching and thinking about justifying the power behind the programs and approaches I will present has caused me to think of about half a dozen new labs and/or demos I can use in my own classroom.*** These will get written up for my presentation and will go into my ever expanding bag of tricks to use.

While writing this I just realized what I'm doing when I prepare a presentation. I reflect on my teaching. Back in ed-college "they" often talked about the power of reflection. Unfortunately I don't carve out enough time to do this, but in preparing to lead workshops or presentations I have to make the time.

*It will be run SE Michigan in the spring of 2010. Let me know if you want more information.

**My physics teacher mentor left me all his books when he retired.

***Damn that's a long sentence. I wonder if it's grammatically correct

Demo of Ohm's Law

As a member of two different local physics groups, DMAPT and the MIAAPT, I get to meet lots of great teachers and more importantly I get to see their best demos. The demo/lab below is not one I actually saw presented, but one I read about in a lab manual written by Dr. Paul Zitzewitz (who recently retired from the University of Michigan, Dearborn). Paul sent me a copy of it so I could mine it for ideas to use in my own teaching.

Ohm's Law is one of those tough ideas to truly understand. The math is pretty easy, but what are voltage, current and resistance really? You can't see them and you can't put your hands on them and so many students never really understand the underlying concepts.

In Paul's lab manual he builds an analogy between electricity and blowing through straws. The straws serve as the resistors. You can use different sizes to represent different resistance values. The pressure you blow with is the potential difference (voltage), and the rate the air flows is current. You can easily extend the analogy by putting multiple straws in series or parallel and "measuring" the effect on current.

Full Length Mirrors

I saw this demo at last summer's (2009) meeting of the American Association of Physics Teachers. It was part of a really great presentation by Dr. Peter Hopkinson. The basic question was, "How large does a mirror need to be to see your whole body?" The concensus response from the audience of physics teachers was, "It depends on how far away you are from the mirror." As it turns out, the wisdom of crowds failed this one.

I managed to score a "full length" mirror from Target for $5. The mirror is one of those cheap ones you put on the back of a door and is nowhere near my height, it's about 2/3 my height. Yet if I mount it appropriately I can see my whole body at any distance. Just watch the video below for full details. Oh, and if you ever get a chance to see Dr. Hopkinson speak at a meeting you should jump at it!

Sound Science Part 2 - Recreate sounds with Audacity

This is one of my favorite demos involving sound. In this demo we use a computer to recreate the sound of a beaker being struck with a pencil or a ruler. All you need is Audacity, which is free. The first part involves the use of the FFT function to analyze our sound and then we use the tone generator to build the sound one frequency at a time.

While sounds are composed of a combination of different frequencies there's more to them than just that. You also have to look at the initiation and the ending of sounds. Specifically for our beaker you have to look at how the various frequencies decay. We can use another feature built right into Audacity to get close enough to the decay as well by applying the "Fade Out" effect several times.

For full instructions, simply watch the video below. If you want detailed written instructions surf over to my page on Probeware on a Budget.

Physics of Superheroes

I've been using pieces of James Kakalios' Physics of Superheroes for a couple years now. This year I'm using it a bit differently. Last summer it was one of two books my honors physics students could choose from for summer reading. Why should english teachers have all the fun? In the Physics of Superheroes Dr. Kakalios doesn't try to explain why superheroes have super powers, instead he focuses on two different questions.

The first is, what are the implications of these powers? In the video below I go through his idea of how fast Superman is moving when he "Leaps a tall building in a single bound". Dr. Kakalios also ask questions like, "Would Ant Man be able to hear us when we talk?"

The second is, if we allow heros to have super powers would they be able to do the things they are shown to be doing? For example, if the Flash can really run as fast as he does could he in fact run across water or up the sides of buildings?

All of the questions and examples used are written from the standpoint of teaching basic introductory physics. His examples are great and my students really enjoy this approach to learning. The book is now in it's second edition and I have to say I'm happy with the changes that were made.

For extra-credit I offered my students the opportunity to write some superhero physics. For more on this or to see the outcomes just go to our class page. They are supposed to be done by the end of Christmas break (2009-2010).

Here Comes Science - Music To Teach

So, maybe I'm living under a rock, but I finally got around to checking out They Might Be Giants' Here Comes Science. I heard about it when it first came out, but dismissed it as being aimed at elementary aged kids. Then I finally got around to listening to an old Science Friday episode sitting on my iPod that featured the collection. While listening I went to Amazon to buy the MP3 album but then Ira mentioned the animations that come on the second disk. I went over to YouTube and sure enough many of the videos are there. Below you'll find the song The Elements. By the time Science Friday was done I'd already ordered my copy. I can't wait to share it with inflict it on my students.

For more music to teach with you might want to check out AstroCappella.

AstroCappella is a marriage of astronomy and music, developed by astronomers and educators and professionally recorded by the rocking a cappella group The Chromatics. Many of the songs, activities, and background science materials are freely available on their site.