You know it’s a good day when physics students put your demo out on social media.
Struggling after yesterday to explain standing waves well, I borrowed the wave driver setup from a colleague. Duh! Why was I going for full-on theoretical when I had a way to make 3rd, 4th, heck, even 14th harmonics on a string?!
I gathered 4th period around the setup and asked what they noticed. They were amazed! “I notice 14 bow ties on the string.” and “I notice a high-pitched noise.” and “I notice these spots on the string that aren’t moving.” We pointed a strobe light at the string and I messed with the frequency of the light to the point of getting a slo-mo string progression within the wave. Then one kid asked what the wave would look like if she snapped a picture of it. I said, “I dunno, give it a try”. Soon, the girl was passing her phone around, amazed at the still image of a standing wave.
Kids wondered what would happen if they doubled or halved the frequency. So we did it.
Kids wondered what would happen if they increased the amplitude, went to a ridiculously high frequency (yeah, their ears regretted that one), or held on to a node. We were able to try all these things.
Vines may have been filmed, Snapchats may have been made. It was all a blur. 30 minutes later, after a little lesson and vocabulary, we had the strongest understanding I’ve ever seen among my students about how standing waves are formed. Kids were calculating frequencies and wavelengths for n-harmonics. I’ve never gotten to this point so fast with a class.
Is it possible that a good demo was really all I needed?
(Based on this picture, you might think I teach only girls. Not true. I teach only freshmen — they still think boys have cooties, so the groups stay well-separated until December.)