## Tuesday, September 13, 2016

### Space Time Cord-inates

This is an example of a little idea that grew, changed and evolved and I'm still not done with it.

A colleague asked for some ideas about free fall and I remembered an activity often called Tin Pan Alley; here is a video demonstrating it. Usually done as a demonstration, hex nuts are tied to a piece of string and dropped from a tall height on to a pie pan or other metallic plate. The sound is better on a thicker reusable pie pan  than the thinner single-use ones. First students are shown a string with the hex nuts equidistant, say 20 or 30 cm. When the string is dropped the sound of each hex nut hitting the pan gets closer to the next hex nut than the last. A second string has hex nuts that are at specific (increasing) distances so that when it is dropped the sounds are equal times apart.

After suggesting this activity to my colleague I began to think about it more and decided to use it in my own kinematics unit. In what felt like a stroke of brilliance I thought of turning this teacher-led demo into a student run inquiry activity. I wanted to hand students ten hex nuts, a pie pan and some string and ask them to determine the distance of hex nuts that would create even interval sounds through experimentation. In the first draft I dashed off I actually titled it "Free Falling Nuts." After remembering I teach in a high school I realized it needed a new title.

But there was a sticking point, how can students be sure that the sounds are in fact even intervals? I decided to try and implement some technology.

I had downloaded the free Physics Toolbox app a few months ago and started to play with its many functions. Its an awesome app I strongly recommend downloading. There is a sound meter on there that records decibel levels over a time axis. I wanted students to use this free app to capture their hex nut hits so that they could compare the intervals between them. I also had Vernier Microphones and wanted to try using them as well. I wrote the whole thing up but before I decided to do it I thought I should try it. Turned out to be a good thing.

My colleague Matt and I decided to try it out before we gave the task to our students. We calculated the distances required for even time intervals with 0.1 s or 0.2 s, etc. and made a few prepared strings. We found that the sounds were so short that neither the Physics Toolbox sound meter nor the Vernier Microphones could pick up the sounds well enough to determine the time intervals. We tried amplifying the sound on a large stool, tried recording and slowing down the recording, etc. Without a 1:1 classroom we didn't want to rely on video analysis. We were forced to abandon the idea of students determine the distances by sound. And given that we wanted to use this as an introduction activity we didn't want students to calculate the distances between the hex nuts yet.

So I was back to the idea of a teacher-led demo. I decided to ask students to predict, discuss and then explain what they were hearing. I wrote this google slide presentation to guide the activity. The background data for calculating the distances for different time intervals is here, first calculated out by Matt. The larger the equal time interval, the longer the string. I was limited to a few meters given my ceiling height, something to keep in mind.

Kevin Healey said...

I've had success with this using Audacity. Washers hitting the table top gave visible spikes using just a MacBook's built in microphone. Your tin foil plates idea is awesome and should only make it easier for Audacity to pick up.

Thanks for reminding me of this one... need to set it up for Friday!

Neat! I like this. I am going to try and adopt this to my AP 1 class next week.

Eric Geggatt said...

Great article! Have you heard of TeenLife? They are a great directory for afterschool programs that I'm sure your students would be very interested in. Check them out!

Jessica Downing said...

I've been doing a variation of this activity for many years--credit to the credential program folks at UCD. I use fairly light weight fishing line and the little pinch-to-open weights. Theoretically this would remove the need for other tools, but pliers are useful, especially after the weights have been around some time.

I give them about 3 m of fishing line to start, permission to stand on the desks/lab tables, and the suggestion that they start by putting the second weight 10 cm from the first. (Most groups then proceed to put all their weights 10 cm apart, resulting in a mush of sound, but they soon figure out what to do next.)

With the fishing line and weights, once on, positions can be changed by sliding the weights on the string. They need to be careful in handling their strings to prevent tangling. As long as I retain right to final approval, most groups come up with a decent placement by the end and follow up by graphing their results to emphasize the non-linearity.

The last few years, some groups have broken out their phones. So far, I've let that happen without formally including it. Some focus on replaying the sound in order to listen repeatedly. Some focus on the visuals, especially effective if they use slo-mo.

On the next day, to make sure all have heard the difference, I drop a line where all the spaces are 20 cm and another with the proper spacing.