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Chapter 5 Wave Motions and Sound

Chapter 5 Wave Motions and Sound. Fig 5.4 (Simple Harmonic Motion). Vibrations. Repetitive, back-and-forth motion Periodic or oscillation Cycle one complete vibration Period time for one cycle Frequency number of cycles per second (units = hertz, Hz)

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Chapter 5 Wave Motions and Sound

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  1. Chapter 5 Wave Motions and Sound

  2. Fig 5.4 (Simple Harmonic Motion) Vibrations • Repetitive, back-and-forth motion • Periodic or oscillation • Cycle • one complete vibration • Period • time for one cycle • Frequency • number of cycles per second (units = hertz, Hz) • Period and frequency inversely related.

  3. Waves • Disturbance that moves thru a medium • Causes • Periodic motion • Pulse • Two major considerations about a wave? • Traveling Disturbance • Transports Energy • Mechanical waves • Require medium for propagation • Waves move through medium • Medium remains in place.

  4. Kinds of Waves Longitudinal waves • Vibration direction parallel to wave propagation direction • Particles in medium move closer together/farther apart • Example: sound waves • Gases and liquids • Support only longitudinal waves.

  5. Kinds of Waves (cont’d) Transverse waves • Vibration direction perpendicular to wave propagation direction • Example: plucked string • Solids • Support both longitudinal and transverse waves.

  6. Waves in Air • Longitudinal waves only • Large scale - swinging door creates macroscopic currents • Small scale - tuning fork creates sound waves • Series of: • Condensations (overpressures) and • Rarefactions (underpressures).

  7. Describing Waves Graphical representation • Pure harmonic waves = sines or cosines • Wave terminology • Wave propagation speed.

  8. Require medium for transmission Speed varies with Inertia of molecules Interaction strength Various speeds of sound. Sound Waves

  9. Velocity of Sound in Air • Varies with temperature • Greater kinetic energy thus sound impulse transmitted faster • Increase factor (units!): 0.6 m/s per °C 2.0 ft/s per °C.

  10. Eq 4.2 HW BONUS Question If the air temperature is 86ºF during a thunderstorm andthe thunder took 2.3 seconds after the lightning was seen. How far away was the lightning strike? Determine the answer in feet and then convert to miles. Then what?

  11. “Visualization” of Sound Waves • Boundary effects • Refraction • Reflection • Absorption.

  12. Refraction Bending of wave fronts uponencountering a boundary.

  13. Reflection Wave rebounding off boundary surface.

  14. Absorption Wave energy dissipated.

  15. Sounds from Moving Sources • Doppler effect • Wave pattern changed by motion of source or observer • Moving Towards - shifted to higher frequency (blue) • Moving Away - shifted to lower frequency (red). Doppler Homepage

  16. Bell X-1 Sounds from Moving Sources • Supersonic speed • Shock wave and sonic boom produced. http://www.wilk4.com/misc/soundbreak.htm

  17. Next Time: Quiz 2 (Ch 4 and 5)then Electricity

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