# ch5 - WAVE MOTIONS AND SOUND Elastic vs Inelastic Elastic...

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» WAVE MOTIONS AND SOUND

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Elastic vs. Inelastic Elastic vs Inelastic
Elastic Materials » When “disturbed” from equilibrium. Restoring force acts toward equilibrium » Carried by inertia past equilibrium to other extreme » This causes a “ Vibration”: a moving back and forth pattern » Period “T”: How long it takes for the pattern to repeat.

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VIBRATIONS » Common in elastic materials ˃ (description coming up) » Some can be seen or heard » Some are invisible » Light and Sound both involve vibrations
Waves » Electromagnetic Waves: (Energy) ˃ Can move through space ˃ Color, radiation » Mechanical Waves: (Require Matter ) ˃ Sound ˃ Ocean » There is a relationship between matter and Energy (E=mc 2 ) But that comes Later

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» How to describe vibrations/waves? + Displacement from the equilibrium position. + A cycle: the movement from some point, to another point and back again. (Repeating pattern) + A period ( T ): time required for one complete cycle . (usually measured in seconds) + Frequency ( f ):the number of cycles per second. Frequency is measured in Hertz (Hz) or 50hz (Pattern repeats 50 times each seconds)
Frequency: » Frequency and Period don’t apply just to waves » AC (Alternating current) Electricity runs on 60hz in U.S. ˃ Ever wonder why outlet need two prongs to work properly? ˃ Electricity comes through one and exits through the other (like water moving turbine) ˃ AC Current changes direction back and forth (50 times per second) ˃ Will look at more in next Chapter » What's the period of the Earth? Time? » Sun? ˃ 24.47days (Equator)

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Describing Vibrations
Describing Vibrations » Amplitude - maximum extent of displacement from equilibrium » Cycle - one complete vibration » Period - time for one cycle » Frequency - number of cycles per second (units = hertz, Hz) » Period and frequency inversely related

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» Waves move Energy!!!!!!

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Waves » Periodic disturbances transporting energy » Causes ˃ Period motion disturbing surroundings ˃ Pulse disturbance of short duration (Ripple) » Mechanical waves ˃ Require medium for propagation ˃ Waves move through medium ˃ Medium remains in place
» Kinds of Waves. ˃ Longitudinal Wave + A wave that travels in a back and forth movement ˃ Transverse Wave + A wave that disturbs particles in a perpendicular motion in the direction of the wave.

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Kinds of Waves Longitudinal waves » Vibration is parallel to wave direction » Particles in medium move closer together/farther apart » Example: sound waves » Gases and liquids - support only longitudinal waves
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• Winter '97
• engrb
• Transverse waves

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