Sound Waves
Thanks to Homeschooled Twins for pointing to this awesome demonstration of the vibrations of sound waves.
Memorizing the Preamble to the Constitution
In preparation for celebrating the 222nd anniversary of the signing of the Constitution of the United States on September 17, 1787, here are a few videos that I hope will inspire you to at least memorize the brief, but powerful Preamble. First, here is my first attempt at video editing with the bare bones Windows Movie Maker (no ability to step through frame by frame with sound, or ability to mark stops before cutting). My daughters are reading the Preamble to the Constitution from the little book I carry in my purse. They can recite it by heart, but it I thought it would be a little different (and a little less exposed) if they did it this way. Let's not even talk about the 20 takes due to laughter. If you knew it as a child, don't worry, it'll come back to you as Barney shows in this video. Finally, a little treat for my sci-fi loving friends. And if memorizing and reciting The Preamble in creative ways leaves you wanting more, you can find out which Founding Father you...
Comments
Kim: Not quite fringe patterns. Here’s the short (!) version:
The square plate you see is being shaken (“excited”) by a motor vibrating at increasing frequencies. When the shaker reaches a specific “natural frequency” of the plate, the plate vibrates strongly (i.e. it resonates). The vibration consists of back-and-forth motion in a specific pattern, or “mode shape”. In each of these modes, some parts (or zones) of the plate are moving up and down, while other neighboring parts/zones are simultaneously moving in the reverse direction (i.e. down and up). The white lines highlight the mode shape, or more specifically the boundaries between the zones; the powder collects here since these boundaries don’t move up and down – i.e. you get something like a trough where the powder collects when it is bounced off the moving zones.
When the shaker vibrates at a frequency between the object’s natural frequencies, the vibration amplitude is minimal, and the distinct mode shapes are not apparent.
A simpler example of this phenomenon is a one-dimensional counterpart to the two dimensional plate above: A vibrating string under tension sways back and forth under excitation at its lowest natural frequency. At the next higher natural frequency, the vibration pattern is a reversing S-shape, i.e. there is one point (in the middle of the S) that is motionless, called a node, while on either side the string moves back and forth. At the next frequency the pattern is a reversing W (two nodes), and so on. Here, the nodal points are the one-dimensional counterpart to the white nodal lines in the two-dimensional plate example above. If you observed the edge of a slice through the plate while it was vibrating, you would see similar S shapes.
Kim, happy to share!
http://www.kettering.edu/~drussell/demos.html
(See the links in the section "Vibrational Modes of Continuous Systems")
Enjoy!