Creating a slow-motion effect in real life takes some creativity, but with the help of a few strobe lights and a frequency generator, the folks at Pasco put together one that’s truly mesmerizing, on top of being educational. Watch the video of it below, and then read on to find out how it’s done!
The lights illuminating the string from below are hooked up to a frequency generator, which turns them on and off several times per second, much faster than the human eye can see. The string is also vibrating up and down at a similar rate, leading to the two-node standing wave that shows up on film as a blur.
Imagine the strobe lights and the string’s vibrations being perfectly in sync, so that the string is in the same position every time the light flashes; the luminous string would appear to stand completely still. When the lights flash just slightly faster than the string’s vibration frequency, the string doesn’t have time to make it all the way back to its previous position before the lights flash again, so the wave appears to have shifted the next time it lights up; the same effect can also be created with a strobe rate slightly slower than the string’s frequency. By keeping the rates very close to one another, the apparatus creates the effect shown above, where the illuminated string oscillates up and down at a rate depending only on the difference between two frequencies. This is actually somewhat similar to the phenomenon of acoustic beats, the odd volume fluctuations you hear when two musicians playing the same note are just slightly out of tune with one another.
Check back soon for more cool demo stuff from the 2015 American Association of Physics Teachers Summer meeting, where we found this awesome science display! (And if you know any “string theory” jokes, let us hear ’em in the comments)