Sound Waves
Thanks to Homeschooled Twins for pointing to this awesome demonstration of the vibrations of sound waves.
Plainsies, Clapsies
A ball bouncing game from my youth instructed the player to throw the ball up ( plainsies ), throw it up and clap ( clapsies ) throw it up and roll your hands ( roll the ball ) and touch your shoulders ( tabapsies ). In trying to locate the rest of the ball bouncing chant, I found out not only is my “tabapsies” a mondegreen , but also the motion – touching your shoulders – isn’t even the correct movement! You are supposed to clap your hands behind your back and say “ to backsies .” Yeah. That makes much more sense. Being only slightly deflated by this discovery, I will still share my exciting news. In an attempt to counteract the stretching of my wrist from doing front squats two days in a row, I pulled out the tabapsies motion this morning. This, in itself, is not newsworthy. However, I grabbed both shoulders with all five fingers!!! Again, not exciting unless you know that when I was nine years-old, I broke my left elbow doing a running c...
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!