AG25_Mechanical_Waves

# AG25_Mechanical_Wave - Chapter 25 Mechanical Waves 2007 Alan Giambattista On a terrible earthquake struck the Hanshin region of Japan killing over

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1 Chapter 25: Mechanical Waves © 2007 Alan Giambattista On January 17, 1995, a terrible earthquake struck the Hanshin region of Japan, killing over 6400 people and injuring about 40,000 others. Some 200,000 homes and buildings were damaged, causing the evacuation to shelters of 320,000 people. The heaviest damage occurred in the city of Kobe, including the buckling and collapse of an elevated highway. However, geologists found that the point of origin of the earthquake was 15–20 km below the northern tip of Awaji Island, about 20 km southwest of Kobe. How did the energy released by the earthquake travel that far with enough energy to cause such great devastation? 25.1 WHAT IS A WAVE? Physicists use only a few basic models to describe the physical world. One such model is the particle: a point-like object with no inner structure and with certain characteristics such as mass and electrical charge. Another basic model is the wave . Water waves are familiar examples. When a pebble is dropped into a pond, it disturbs the surface of the water. Ripples on the surface of the pond travel away from the spot where the pebble landed. This traveling disturbance of the water surface from its equilibrium position is a wave. Any wave is characterized as some sort of “disturbance” that travels away from its source. Chapters 25 and 26 concentrate on mechanical waves traveling through a material medium, such as water waves, sound waves, and the seismic waves caused by earthquakes. Particles in the medium are disturbed from their equilibrium positions as the wave passes, returning to their equilibrium positions after the wave has passed. Chapter 27 discusses electromagnetic waves such as radio waves and light waves, in which the disturbance consists of oscillating electromagnetic fields. Two of the five human senses are wave detectors: the ear is sensitive to the tiny fluctuations in air pressure caused by compressional waves in air (audible sound) and the eye is sensitive to electromagnetic waves in a certain frequency range (visible light). Waves Can Transport Energy Suppose we drop a pebble into a still pond. The kinetic energy of the pebble just before it hits the pond is partly converted into the energy carried off by the water wave. That waves carry energy is clear to anyone who has been surfing or swimming in the ocean. Surfing the web is also made possible by waves. Information on the Internet is carried by waves of various sorts: electromagnetic waves in cables, microwaves between Earth and communications satellites, light waves in optical fibers. Microwaves in ovens carry energy from their source to the food; the electromagnetic energy of the microwaves is absorbed by water molecules in the food and appears as thermal energy. Electromagnetic waves from the Sun bring the energy that makes life possible.

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## This note was uploaded on 03/27/2008 for the course ECON 1110 taught by Professor Wissink during the Spring '06 term at Cornell University (Engineering School).

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AG25_Mechanical_Wave - Chapter 25 Mechanical Waves 2007 Alan Giambattista On a terrible earthquake struck the Hanshin region of Japan killing over

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