Physics Principles and Problems 2009

Chapter 14: Vibrations and Waves

Problem of the Week

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Wave of Destruction
On average once a year, somewhere in the world a wave of giagantic proportions strikes a coastline. These monster waves are caused by earthquakes that raise or lower the seafloor. A shifting seafloor can elevate a California-sized chunk of earth 30 feet in just a few minutes causing an enormous amount of water to move, stretching from the seafloor all the way to the surface. This disturbance sends ripples across the ocean that can propel ashore 100,000 tons of water for each five feet of coastline.
If you are close enough to see a tsunami, you won't be able to out run it, guaranteed.

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Tsunami
<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg::::/sites/dl/free/0078807220/193800/ch14_waves.jpg','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (9.0K)</a> Tsunami is the wave of destruction. Moving across the deep ocean however, a tsunami is essentially invisible. It may be 20 feet high, but its wavelength might be 200 miles; This means that the slope of the wave averages about two inches each mile. Herein lies the destructive nature of the tsunami. With a long wavelength a tsunami travels thousands of miles with little loss of energy. The wave of a tsunami may take 24 minutes to rise and fall, thus traveling across the ocean dissipating little energy as it goes. Tsunamis are so efficient and persistent that they can reverberate through an ocean for days, bouncing back and forth between continents.
<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078807220/193800/POWproblem1.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a> Using the information given in the text, determine the speed at which a tsunami can travel.
 
<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=gif::::/sites/dl/free/0078807220/193800/POWproblem2.gif','popWin', 'width=NaN,height=NaN,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (0.0K)</a> Before a tsunami strikes, a drop in sea level occurs. Right before the Nicaraguan tsunami of 1992, the boats in the harbor of San Juan Del Sur crashed into the bottom as the water level dropped some 10 to 20 feet. In Hawaii in 1946, the bizarre sight of a draining harbor attracted onlookers who were then drowned a few minutes later when a tsunami rushed in. Explain the mechanism by which water retreats before the oncoming wave.
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