# Bungee Physics

Last week, Paul, Ed, and I did physics. This is such a rare event that it deserves note. We actually developed a theory, collected data, compared theory to data, came up with new ideas and tested them. We only wish kids everywhere could have the same experience.

This investigation was prompted by Ewa Kedzierska’s presentation at the World Conference on Physics Education in Istanbul in early July*. She presented a student activity on bungee jumping that claimed that the jumper falls faster than a free-falling object. This seems difficult to believe, in spite of video data she presented—collected and graphed by the wonderful COACH software—that clearly showed this to be true. We immediately thought of many reasons why this should be impossible. Imagine jumping without a tethered Bungee cord—jumper and cord would fall in free-fall just as Galileo proved in his famous Tower of Pisa experiment (never mind the fatal consequences—this is physics!). Attaching the far end of the Bungee rope would seem to apply an upward force that could only slow the jumper, not speed her up!

As typical science skeptics, we had to do it ourselves and understand the mechanism, if the effect was true. Following the maxim that was current when CERN supposedly found neutrinos travelling faster than light—“Extraordinary results require extraordinary evidence”—we needed to do the experiment ourselves and get a feel for the situation. So Ed  gathered a stepladder, chain (substitute Bungee), tennis ball (for the jumper), and a camera that takes 240 frames per second, and we collected data.

Paul, ever the theoretician, showed that the far end of a horizontal chain link held steady at the near end would fall faster than a free body, and hence, could impart some force to the falling chain. Thus, each chain link, on reaching the bottom of the “U” formed by the falling links, could impart a bit of force on the falling side and make it fall faster than free-fall. Another way of saying this is that each link, when brought to a halt, rotates 180 degrees and can exert some torque on the falling side.

We collected the data, and clearly saw the effect. It is real! And it is huge when the falling mass is small. We photographed side-by-side tennis balls, one attached to a chain and one in free fall. The one with the chain fell faster! Every time. The picture shows a frame from a movie of the experiment, clearly showing Paul about to fall (he didn’t), and the free-falling ball going slower.

Don’t believe us? Do it yourself. We attached a force sensor to the end of the chain and could detect the force from individual links. The force increased non-linearly and dramatically. Stopping the last link required 50 N even though the entire chain weighed only 4 N (see graph). We are still arguing about why the force increases so much for the last few links.

I noticed that sometimes if the falling part of the chain is close to the tethered part, the links at the bottom of the “U” do not rotate, but slide. When they slide, they do not rotate and, hence, should not accelerate the falling chain. We could hear the difference, but our results were inconclusive, because near the end of the fall, the chain doesn’t fall evenly and this causes it to revert to the link-rotation mode.

In our next blog, we’ll present the data and our analysis. Stay tuned.

# Streaming Arduino Data to a Browser without Flash or Java

This blog post describes an older method for connecting sensors to a web browser. You can learn about a newer, more robust method using Web Bluetooth in this Under the Hood article from our fall 2017 @Concord newsletter.

What if you were reading a blog or working through an online lesson and you could just plug in your Arduino and start taking data or interacting with models right in your browser?

Here at the Concord Consortium we are very interested in making sensors that are easy to use in the classroom or embedded directly into rich online curriculum. We’ve done some work in the past using applets as an intermediary to read data from commercial sensors and displaying them in lightweight graphs in the browser. When we think of fun, hackable, multi-probe sensors, though, we naturally think of Arduinos — we are open-source geeks after all.

In thinking of ways to display Arduino data in a browser with the minimum amount of fuss, we considered both our existing applet technique and using the new HID capabilities of the Arduino Unos. But while we will probably still find uses for both strategies, it occurred to Scott Cytacki, our Senior Developer, that we could simply use the common Ethernet Shields (or the new Arduino Ethernets) to send the data directly to the browser.

With this idea, it was quick work to hack the Arduino Server example to send JSON values of the analog pins and create a webpage that would rapidly poll the Arduino for data. So here is the first example that I wrote in about 70 lines of code (including the Arduino sketch) usable on any Ethernet-capable Arduino on any browser:

2. Plug in your ethernet shield, connect the Arduino to your computer with an ethernet cable and wait about 30 seconds for the Arduino server to boot up
3. Optionally connect a sensor to pin A0. (The demo below is scaled for a L35 temperature sensor, but you don’t need it — you might need to rescale the graph by dragging on the axis to see the plot, though)
4. Click the “Start Reading” button below

You should see your Arduino data filling up the graph. If not, wait another 20 seconds to ensure the server is fully booted and click the “play” button at the top right to start it again.

Wow, that was actually pretty easy!

I created the slightly more complicated example below reads data from all six analog pins, applies an optional conversion, and graphs any one of the data streams. If you were already reading data above, you don’t need to do anything new, just hit the button:

We think this is really cool, and we can’t wait to come up with new ways to integrate this into online content. Why not feed the temperature data into the HTML5 version of Molecular Workbench we’re developing under our new grant from Google.org, for instance, and see the atoms speed up as the temperature increases? Or set up an online classroom where students across the globe can take environmental readings and easily upload and pool their data?

Even by itself, the example above (perhaps expanded further by an interested reader) makes a great workbench for developing on an Arduino — much better than watching the raw Serial Out panel. And of course all the programming can happen in your friendly JavaScript environment, instead of needing to keep recompiling code and uploading it to your Arduino as you iterate.

### Technical details:

• This Arduino Sketch creates a server running on http://169.254.1.1, which is a private local IP that will automatically try to not conflict with other servers, allowing for easier connection without a DHCP server. The sketch then returns JSON data using the JSON-P technique of calling back a function, which allows us to make cross-domain requests.
• Click on the tabs at the tops of the embedded jsFiddle examples to see the source code for streaming data to the webpage, or fork and edit any of the examples yourself.

• The graphs are creating using D3.js, and make use of the simple-graph library created by Stephen Bannasch.

# Why Aren’t There Probes in More Classrooms?

Bob Tinker, Emeritus President of the Concord Consortium, noted, “The creation of probeware represents one of the most valuable contributions of computers to education.”

In 1981, Robert Tinker and Stephen Bannasch from the Technical Education Research Center developed the first educational temperature grapher. This software was developed for the Apple II computer and was part of the National Science Teacher Association Project for Energy-Enriched Curriculum funded by the U. S. Department of Energy. HRM (Human Relations Media) Software in early 1983 published the software developed in the PEEC project, including the Apple II Temperature Grapher. Many other types of probeware followed in the coming years for use on many different microcomputers called MBL (microcomputer-based labs) and later for the CBL (calculator-based labs).

Today, a huge variety of inexpensive probes is available. Probe software is used on laptops and mobile devices in schools. Probes provide opportunities for students to collect and display data immediately that they normally can’t see, such as graphing friction shown below for a box containing different masses.

Friction of a box on carpet containing different masses, Collected by Ed Hazzard, June 2011

Probes often collect hundreds or thousands of readings per second and the visual graph of the data shows changes as well as overall trends. In 1987 Heather Brasell’s dissertation from the University of Florida research compared traditional paper-and-pencil graphing methods with the instantaneous computer displays equipped with sensors. She found that students have a significant increase in retention of graph understanding when they see the graph instantaneously while the data is being collected.

In addition to short investigations, students can record and display data collected over long periods of time, some even up to a year. Software also can display simultaneously on the same graph collection from multiple probes. This same software can use the data collected from two or more different probes to provide a derived display – like displaying electrical power while students use voltage and current probes.

Although it has been thirty years since their introduction, not every science and math classroom uses probeware. Since probes hold so much promise to help students investigate and learn about the world around them, the big question is: why not?