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Think about dropping a stone into a pond on a very calm day. As the stone breaks the surface of the water, the surface oscillates up and down—in harmonic motion. But something else happens to the water surface: Ripples form and spread out. Everything the ripples touch also oscillate up and down with the same frequency. An oscillation that travels is a wave, and waves are the subject of this section. Both sound and light are waves. There are even gravity waves created when black holes crash into each other. 
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The importance of waves
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Waves are an essential way by which energy travels. Think about the example of a stone falling into a pond. The ripple causes the water and objects floating in it to move up and down some distance away. Where did their energy of motion come from? The answer is that it came from the stone and was carried by the wave. When the stone hit the water surface some of its kinetic energy was converted to waves. The waves spread out over the surface of the pond, dispersing the kinetic energy of the stone over a much broader area of space than was directly touched by the stone itself.
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A fundamental reason for why waves are important is that any disturbance that releases energy often produces waves. The waves spread the energy out and propagate the disturbance through space, affecting other regions, which may be quite far away. A wave pulse on a long spring is a good example. To make a wave pulse on a spring, disturb one end by rapidly jerking it up and down once. The disturbance quickly moves away from your hand and travels along the spring. Areas of the spring far away from your hand are affected as the wave pulse reaches them. As the pulse moves, the energy of the disturbance is spread along the spring and also dissipates through friction. The wave pulse gets smaller until the spring is at rest again.
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This aspect of waves is frighteningly displayed by earthquakes. In an earthquake a tremendous amount of elastic potential energy is released when stressed rock deep underground suddenly slips and realigns itself. That energy is largely released as seismic waves that oscillate the ground up, down, and sideways. Just as a wave pulse moves along a spring, seismic waves race away from the earthquake epicenter at the speed of sound. When the seismic waves interact with matter, energy can be released—which can topple buildings 100 miles away during a powerful earthquake! Architects must design buildings to withstand the energy from oscillations (or shaking) caused by earthquakes.
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