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Low-Cost Wave Generator Apparatus

posted Jan 20, 2017, 11:16 AM by Mark Schober   [ updated Jan 20, 2017, 12:49 PM ]

There's something magical about standing waves, resonance, and the tangibility of the nodes and antinodes. Students love working with the waveforms, and they love the direct connection to the musical instruments they play, how harmonics work, and the physical principles behind them. Unfortunately, most science suppliers sell a wave driver apparatus in three parts: 1) the thing that does the shaking, 2) an amplifier, and 3) a function generator. Depending on the supplier, one full setup costs between $500 and $1000. That's a bit steep for me, so here's a homemade version that costs about $30 in parts and takes advantage of the capabilities of your smartphone.

Mechanical Waves Make-n-take Workshop Description

This workshop is based around a low-cost wave-driver and amplifier driven by a free smartphone app for investigating wave propagation in strings. Similarly capable equipment from science suppliers ranges from $500-$1000 per lab setup, while ours costs just $30. We will begin by using the wave driver to examine properties of standing waves on strings, followed by a lab to quantitatively investigate how string tension and string mass per unit length affect the wave speed. You will then build your own wave drivers to use with your students. Not only does this set of labs establish the fundamental properties of mechanical waves, it provides an excellent experimental environment for students to collect and analyze data for a phenomenon that depends upon multiple variables. 
I'm leading a make-and-take workshop to build these for your classroom on March 5th, 2017, from 10 am - 1pm. If you are available to join me at Teacher's College, Columbia University, I'd love to see you there. Registration for the workshop is $20, and wave drivers are $30 each. You can sign up for the workshop here:

In the workshop we will run the labs you can do with this equipment as well as build the apparatus.

Design Features:

The apparatus consists of a 4-inch speaker, a 50 Watt mono amplifier circuit, a 12-volt power supply, and some laser cut acrylic pieces that tie things together. All parts are mounted to a central acrylic plate that includes notches for cable management when not in use.

You need to supply a ring stand with a flat metal base. The metal rod acts as one anchor for the string and the acrylic pyramid mounted to the speaker cone shakes the string. The magnet in the base of the speaker sticks nicely to the metal base, and a hole in the acrylic plate locks it onto the ring stand rod. This makes the overall apparatus much heavier so that it doesn't go wandering all over the lab table.

There are a variety of smartphone apps that feature tone generators or frequency generators for free. I've been using one called "Function Generator" on an iPhone which works fine, though it's filled with pop-up ads. You want a program that allows you to increase or decrease the frequency by tapping add 1 Hz or add 10 Hz. Interfaces dependent on a slider or having to type in the frequency are difficult to use for this application. I want to give a shout out to the Physics Toolbox app that gives you access to all of the sensors in your phone. (Check it out!) The Physics Toolbox has a tone generator for Android that allows you to change frequency by tapping, but it does not work that way on iOS.

There isn't anything that complicated here that you couldn't do with a saw and a drill, but once I refine my prototype for the workshop, I'll post the cutting templates and a detailed parts list here to save you some time. I'll also share the curriculum materials that go with the lab and the followup analysis.

Soda Can Photoelectric Effect Demo

posted Apr 8, 2016, 6:55 PM by Mark Schober   [ updated Apr 8, 2016, 8:17 PM ]

The photoelectric effect was at the heart of not just one Nobel Prize, but two! If you can plunk down $175 for a short wave UV source, (or borrow the one your biology teachers might have around for looking at bacteria) you can visit the grocery store to get everything else you need to demonstrate the photoelectric effect.

I'll also tout the soda can electroscope, which, for the low-low cost of dumpster diving, is superior to commercial electroscopes as far as I'm concerned. The pie-plate electrophorus is also as good or better than any commercial one on the market. Just imagine what Ben Franklin might have discovered with access to the wonders of modern take-out containers. As it was, he had to do with pewter and wax -- it was definitely a different time.

Click the image below for instructions on how to assemble the apparatus and do the demo.

LED Photoelectric Effect Apparatus

posted Apr 5, 2016, 5:35 PM by Mark Schober   [ updated Apr 5, 2016, 5:35 PM ]

I first encountered Wayne Garver's low-cost LED photoelectric effect apparatus at a St. Louis Area Physics Teachers workshop in 2005. Since then, I've been involved with building dozens of these. This latest version has some simplifications and additions that make it even more useful. Wayne's insight is that the essence of the commercial apparatus is the vintage phototube at the heart of the device. All the other parts are pretty simple and inexpensive. So for about $100 in materials, you can build an apparatus that will do everything that the $500 version in the science catalog will do.

In the fall of 2015, my engineering classes built 30 photoelectric effect devices as an orientation to using tools and soldering. Some of the solder joints aren't pretty, but they all have been tested and work fine. It was a great mix of practical skill development and service to the school and the science teaching community. At the upcoming workshop, we're selling the apparatus for the cost of the parts.

In April 2016 I am offering a workshop on the photoelectric effect and how to get the best learning impact from the device. If it isn't yet April 9th, sign up to join us!

The light source for the device is a set of LED's. Since all of the LED's are clear, it is necessary to label them. I was ready to pull out my model railroad paint when one of my workshop participants in 2014 suggested using fingernail polish. What a brilliant solution!

Here's a close up.

Here are the directions I've written up for constructing the apparatus. Click the image to open the pdf.

I'll add a few more photos and resources after the April 9 workshop.

Slow Acceleration Apparatus

posted Jul 29, 2015, 12:02 PM by Mark Schober   [ updated Apr 8, 2016, 8:27 PM ]

So that students develop a real kinesthetic sense of accelerated motion, it is ideal for them to investigate the acceleration of an object that speeds up so gradually that the motion can be tracked by hand. Rex Rice, of Clayton HS in St. Louis, developed an elegant solution using a wood disk with an axle made from two golf tees. The wheel and axle rolls down two conduit pipes, and students can mark the position of the axle at equal time intervals, getting 20 to 30 measurements depending on the incline.

After making hundreds of these -- it's no small task cutting 4" disks out of MDF and drilling centering holes in the end of the golf tees so that they align appropriately -- Chris Doscher suggested to me that we could use CD's for the disks by using a faucet washer to attach them to a metal axle. The metal axles are less ideal because they don't have the self-centering feature of the tapered golf tees and the friction between the steel axle and the steel pipes is low enough that they have a tendency to slide on the pipes. One fix we have added is to run a strip of masking tape down each pipe to give a bit better tooth. However, we sometimes found that the disk's acceleration decreased as the wheel approached a terminal velocity with the tape -- be sure to test it ahead of time! I haven't tried it yet, but I want to spray the axle with a clear lacquer to change the friction characteristics.

Slow Acceleration Apparatus Instructions
Click to download a .pdf of the directions to build the apparatus.

The teacher's notes for the Uniformly Accelerated Particle Model on the AMTA site spell out the details, but here is an outline of how you could use this apparatus in an instructional sequence:

1. Introduce the activity in terms of prior explorations in constant velocity. Our goal is to quantitatively describe the motion of an object that has a changing velocity. Students will recognize that they can use the same data collection procedure that they used in the constant velocity lab.

2. Students collect data for the moving disk.

3. Students make a position-time graph. Since it's curved, guide them to the idea of finding velocities at different times of the motion by drawing tangents to the curve and finding the slope.

4. Students make a velocity-time graph. In the discussion, acceleration is quantitatively defined.

5. Students are challenged to make and analyze additional graphs: "Linearize" the position-time graph by making and analyzing a position-time squared graph. Graph velocity vs. position and linearize the graph. By the time the analysis is done, all of the accelerated motion equations have been developed from the data.

Mega Can-Crush

posted Jul 13, 2015, 9:42 PM by Mark Schober   [ updated Jul 13, 2015, 9:55 PM ]

I've done the can crush lab with my students for years, but only crushing soda cans. I find this image of a crushed railroad tank car fascinating and I wrote a quiz based on it for use in Modeling Chemistry Unit 3. By the way -- the steel that makes up the tank on a DOT-111 tank car is 7/16" thick and has a capacity of 34,500 gallons. Because the DOT-111 design has been involved in several recent oil train accidents, the DOT-117 design will replace it, with 9/16" thick tank steel and full end shields. As a railfan, I follow all kinds of information about trains, and I also came across this on news feed:

From on July 7, 2015:
Kelso Technologies Inc. obtained approval from the Association of American Railroads to begin commercial field trial testing on the company’s new Vacuum Relief Valve. The low-pressure device is specifically designed to protect tank cars from the effect of an excessive vacuum, preventing the implosion of the tank car. The patent-pending valve design is a result of customer demand for a better performing product due to the failure rate of products currently in use. The device meets the new DOT-117 tank car specifications to be implemented later this year. Kelso is a railway equipment supplier that designs, produces and sells proprietary tank car service equipment. For more information, go to

Upgrading an old website

posted Jun 27, 2015, 6:46 AM by Mark Schober   [ updated Jul 16, 2015, 8:50 PM ]

Dear friends, I started this website over 15 years ago as a resource for my students and, as it turns out, for the wider Modeling community.

I have had the opportunity to teach physics in grades 9-12 in public and private schools, and implementing the Modeling Instruction philosophy with each combination of variables required customized curricular approaches to best fit my students. I'm now at a school that has its own password-protected web system and maintaining multiple web platforms has been too much to keep up with. Therefore, some upgrades are needed.

This site is now designed specifically for teachers. My students can get what they need through my school site. This tighter focus will make the site easier to use.

Most of the physics materials I've created and the resources I've assembled are available through the American Modeling Teachers Association website. They have a great platform for exchange of ideas that compiles the resources and brains of hundreds of teachers throughout the Modeling Instruction community.

Many bloggers have done an excellent job of addressing questions and providing excellent resources for teachers. I will add to the blogosphere when I have something to say, but I will also happily refer you to the excellent thinkers out there who have stated things beautifully. My own writing has also appeared on the AMTA site and on the STEMteachersNYC site.

I've branched out beyond physics. I've now taught Modeling chemistry, environmental science, astronomy and modern physics, and engineering. I also have a dozen years of experience with middle school astronomy and meteorology teaching. Along the way I've learned a lot and have generated some resources that may be worth sharing.

I've also thought a lot about developing storylines that connect the entire science sequences together by considering what is central to the scientific subdisciplines and what will set students up for subsequent studies and science. When considered in a holistic way, the physics first sequence makes perfect sense and Modeling approaches in physics makes it doable.

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