PHYSICS 8A-08 (04-1-10) - PHYSICS 8A Professor Joel Fajans...

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PHYSICS 8A Professor Joel Fajans 4/1/10 Lecture 19 ASUC Lecture Notes Online is the only authorized note-taking service at UC Berkeley. Do not share, copy or illegally distribute (electronically or otherwise) these notes. Our student-run program depends on your individual subscription for its continued existence. These notes are copyrighted by the University of California and are for your personal use only. D O N O T C O P Y Sharing or copying these notes is illegal and could end note taking for this course. LECTURE We left off last time discussing resonance. In particular, we discussed resonances and what can go wrong. There is another famous example of destructive resonance in physics. The Tacoma Narrows Bridge was one of the longest suspension bridges. The wind was capable of exciting the motions inside the ridge. There was a vibrational mode which was easily excited, and the bridge lacked proper stiffening and so when the wind blew at the proper angle, the bridge started vibrating at its resonance frequency. These vibrations got to be larger and larger until spectacular oscillations led to its collapse. We are moving on to a new topic. We are discussing oscillations which move, or waves. For example, I can send a wave bouncing back and forth through a rope by snapping it. Waves are ubiquitous in nature. For example, I am communicating to you via sound waves. Light is a wave. In biology, you can model the propagation of an electrical impulse down a nerve cell as a wave. The simplest waves to visualize are waves on a rope. There are many different types of waves. For instance, if I pull the wave, the point in the center moves left and right. A wave which has the oscillation in the direction it is going is a longitudinal wave . Sound waves, for example, are longitudinal waves. Transverse waves are waves which occur when you pluck the rope. It is a displacement of the rope up and down, transverse to the direction the wave is propagating in.
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