Today I had a rather fun run in with freshman physics.

I was helping one of my colleagues with her experiment when she asked me for something that could reach far into a narrow vacuum chamber with a piece of sticky adhesive on the end. She needed a way to pick up a sample but we dared not just try to reach in and grab it or to put in a mechanical arm that could easily scratch some of the delicate hardware.

I thought for a moment and decided to get a meter length of quartz-glass rod. We have plenty in the lab since I frequently use quartz in my experiments and it can be easily made clean enough to be placed in a vacuum chamber. It worked well and we retrieved the sample, but what was interesting was what occurred while I was cleaning the quartz. I was wiping down the quartz with a small clean cloth with a bit of methanol. The rod began to sing much like a wine glass rings when the rim is rubbed (or a tuning fork after being struck). It had a clear resonant frequency and surprised me with both the clarity of the sound and how quickly it became loud. So, this kind of thing wasn’t exactly new to me. But it was interesting enough to do a little thinking.

First, the sound comes from vibrations of the rod that build up. The origin of them comes from the friction of the finger/cloth down the rod (or rather the rapid and steady slipping and stopping that occurs due to the friction). Now, this kind of thing is known as a “forced-damped oscillator”. It’s kind of a way of saying that you have an object that can vibrate around so equilibrium position (the rod bending and compressing), that the object has something continually “pushing” it out of equilibrium (the slipping cloth), and that it has some ways of dissipating the energy (in this case the vibrations causing the sound you hear in the air). If you put all this together what you get is an object that can begin vibrating a great deal when the energy being added comes in at or near the right frequency.

Now, the next few questions follow rapidly. What is the frequency? Well, the speed of sound in quartz is roughly 540 meters/sec and I had about a meter in length. The next important thing is that I was holding the rod at the mid point. This lets the rod vibrate on each side of where I’m holding it. So, 1/2 the wavelength on each side. What this boils down to is that the rod was ringing at roughly 540 Hz. That’s somewhere between C and C# one octave above middle C. The next game is to try subdividing it further. Grasping the rod at a point 1/4 down the length (half way between an end and the middle) I moved the cloth down the rod. Pretty quickly I got a new note that was twice as high in pitch (frequency) as the first one. Likewise suspending it on the ends where the entire rod could vibrate (at 1/2 the total wavelength) got a dull note that was near middle C. Holding the rod at other divisions didn’t produce anything audible. Also holding the rod at positions not clearly subdividing the length into “regular” portions produces no sound. What this can tell you is that the vibrations of the quartz is “transverse” to the length. Or in other words, the rod is vibrating at a resonant frequency by bending side-to-side. It could also be vibrating along the length of the rod (sections alternatively compressing and expanding in that direction).

Anyhow, it was a nice thing to run across and made for some physics 1 fun.

And if you ever want to really see how important this kind of thing is, check out the original Tacoma Narrows Bridge. In particular be sure to see the video.

We start another experiment tomorrow at the Advanced Photon Source. Our aim is to look for some special kinds of ice as liquid water freezes in some novel circumstances. It’s an ambitious experiment, but should be really nice if we get a positive result.

One of my simple pleasures in life is seeing how different people react upon first seeing a celestial object through a telescope. So late yesterday afternoon I took a break from faculty applications, proposal writing, and research papers to instead drag my old rickety Dobsonian telescope out for Halloween night.

Currently Jupiter is rising in the early evening. In fact, given the circumstances I don’t think I could have asked for a better object. I got everything set up a little too early so as kids and adults would come by I was some “very strange guy” with a giant tube on the sidewalk. I’ll admit that my homemade telescope is not a beauty queen and in fact looks rather like a water heater. So yes, I was the guy in a tie-dyed lab coat messing with his water heater out on the lawn on Halloween (I may have in fact frightened some children or at the least made some adults uncomfortable). You can tell people it’s a telescope and not a water heater (or some other piece of junk), but you really need something to look at before being taken seriously.

Eventually though the sky grew dark enough that I could find Jupiter (it being actually the first “star” I was capable of seeing). Very quickly after that my situation changed. There was jupiter; cloud bands clearly visible, colored white, browns, pink and orange. The great red spot was not visible, but the 4 largest moons made a nice decoration. At this point the “wow” factor took off. Anyone that I could convince to look through was suitably impressed and amazed. I had one little girl that came back 4 times bringing others with her to see the great planet. And that’s the whole reason for bringing the telescope out. No one expects that peering into a black eyepiece on some giant white cardboard tube is going to show them anything special. Instead of just seeing a little spec of light and being told “that’s jupiter” they instead see the full disk of the planet, enough structure to make out some detail, and its own moons.

And yes, I did my part to combat dentistry as well.