Created by Dale Olive and Randy Scoville
Future Flight Hawai`i instructors
Demonstration at a glance:
A short solid aluminum dowel is stroked causing it to emit a very high pitch and loud sound.
Visit the music store and buy a bar of violin or cello rosin. This is used to create "slip/stick" on the strings as the bow is pulled across it thus vibrating them. Rub the rosin bar on the solid aluminum dowel to give it a good coating. Find the center of the bar by balancing it on your finger. Holding it there tightly with two fingers, grip the bar with two or three fingers on your other hand and pull away from the center. By repeating this over and over the bar should begin to resonate.
The cello rosin allows your finger to slip/stick on the aluminum rod much the same as it allows the bow to slip/stick on the violin or cello. This motion gets the bar vibrating. When the vibrations reach one wavelength, the bar will resonate. Resonance amplifies the amplitude, the height, of the wave which is directly related to the volume we hear. By holding the bar in the middle you are holding it along the node, where the wave passes through the bar, thus not interfering with the wave allowing it to grow.
I usually show a short video on the Tacoma Narrows bridge, also known as "galloping gerdy" when I do this demo. Engineers poorly designed the bridge in such a way that the wind got the bridge resonating causing it to vibrate so much that it destroyed itself. Resonance is also why the Army breaks step when they come to a bridge. By marching over it in step they may actually get it resonating.
Communications: Hawaii Space Grant Office
This activity is featured in Future Flight Hawaii, a K-12 Education Project of Hawaii Space Grant Consortium.
FEB 27 2001.