Getting to “I don’t know”

I like Isaac Asimov’s quote about Science:

The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ (I found it!) but ‘That’s funny …’

I tell them repeatedly that all the interesting stuff in science is the unexpected. When things don’t quite turn out how we thought it doesn’t mean its wrong – just that there is more going on than the oversimplification of textbooks would lead us to believe. I love it when we encounter something odd – for me, the best thing that can happen in a Science class is the students push the boundaries of what I can explain, so we have to learn together. I love getting to “I don’t know”.

Today, we were examining resonance in a tube open at both ends, when it resonates after being “bopped” on one end. Here is the spectrogram:

Frequency is on the vertical axis, time horizontal, and intensity is the brightness. Hitting this tube clearly produces frequencies of 200 & 400 Hz. The fundamental frequency of a 40 cm tube open at both ends is 400Hz. So where does the 200Hz sound come from?

Ah, well, hitting the tube on one end temporarily makes it closed at that end, so it should, at that time, resonate at 200Hz. Fine. But why does it continue to resonate at 200Hz? The 200Hz resonance clearly continues well past the peak of the 400Hz resonance, and yet the tube should not be able to sustain it with both ends open. So why does that happen? I don’t know.

Isn’t Science cool?

2 thoughts on “Getting to “I don’t know”

  1. John Paul

    I really enjoyed your post. I’m a big Asimov fan and hadn’t seen that quote before. I’m also a Physics teacher and was really impressed with the cool result your class found. I’d be interested in knowing what you used to get the spectrogram… Very impressive. I’ve just started following your stuff on twitter. Best wishes, John-Paul


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