A Slinky Saves the Day

About a week ago, Derek Muller ( of Veritasium Science Videos) posted a series of three videos on what happens when you drop a hanging slinky. The videos use some high speed video footage to observe how the lower end of the slinky hovers (quite unintuitively) in place as the top of the slinky falls. The videos first came to my attention Thursday morning when Frank Noschese tweeted that his students were all over the video, wanting to do their own slinky drop. Later that morning, Rhett Allain posted a Vpython computational model of a falling slinky… cool!

Now Thursday was a tough day for me. My students in my second year physics class had done a poor job on a problem set and a “meh” job on a test, and it was clear I needed to give them a “pep talk” about level of effort and seriousness of purpose. To complicate matters, I was in the throes of a bad cold, leading to several nights of insomnia. I was grouchy and did a poor job of “inspiring” my students in class when I spoke with them about their poor performances. They were sitting there, looking like puppies with their tails between their legs, so I said, “Ok, enough of that… you know what to do in the future. Let’s do something fun now,” and we watched the slinky drop videos. And we dropped a super long slinky ourselves from the top of the science building. And we started discussing the physics, which led some to propose using Tracker to analyze the motion. I realized that if we wanted to do a good job of analyzing the video, we needed the raw high speed video of the drop and something in the video to set our scale. A quick tweet to Derek resulted in all the information we needed by Friday morning.

So Friday rolls around, and I have my other section of advanced physics. They are ahead of the class I had Thursday and that other class was going to be mostly missing for Saturday’s meeting due to SAT’s. So I decided to let everyone loose on Tracker with the high speed video. Most of the class was spent just learning the software, but a few students were able to confirm Rhett’s computational model (see the comments on Rhett’s blog).

Now I’m two days behind my syllabus, but I think my students are far ahead of where they were on Wednesday. The discussions we’ve been having about forces on various parts of the slinky are detailed and nuanced in ways that my students’ problem sets weren’t. We’ve just finished talking about Young’s modulus and the speed of sound in solids, so this is an excellent “weak spring” example for them to ponder. And, in a huge win, I convinced at least one student to post a comment on a blog. I tried to get them to email Rhett and Derek, but they are too shy for that. Soon, maybe.

In addition to energizing my second year physics class, the slinky work gave me something to do with my Friday afternoon first-year class, which had been decimated by early sports dismissals. While the six students in attendance did not do video analysis or read Rhett’s blog, they did get to drop a really, really long slinky from the third floor in our main building (in the stairwell, right near the main entrance, where we captured the attention of a bunch of freshmen and some visitors… win!). And they were able to discuss what must be true about the forces on the top and bottom loops of the slinky (after I told them that the video analysis shows the ends have constant velocities). The Balanced Force Particle Model pays off in the real world.

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