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Chapter 15

Chapter 15. Wave Motion and Sound. Objectives. Describe characteristics of mechanical waves Describe electromagnetic waves and the electromagnetic spectrum Analyze sound waves and explain the Doppler effect. Wave Characteristics. Energy may be transferred by wave motion

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Chapter 15

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  1. Chapter 15 Wave Motion and Sound

  2. Objectives • Describe characteristics of mechanical waves • Describe electromagnetic waves and the electromagnetic spectrum • Analyze sound waves and explain the Doppler effect

  3. Wave Characteristics • Energy may be transferred by wave motion • Sun's energy, radio waves, light from lasers • Source: something that vibrates • Frequency of vibrating source = wave frequency • Wave: disturbance that moves through a medium or through space • Displacement of atoms from equilibrium, pulse in spring, change in light intensity • Transfer of energy in direction of travel

  4. A seagull floating on the ocean moves up and down as waves pass.

  5. Wave Characteristics • Pulse: nonrepeated disturbance that carries energy through a medium or space • Transverse wave: disturbance where motion of particles is perpendicular to direction of wave motion (figures 15.2, 15.3) • Example: water waves • Longitudinal wave: disturbance where motion of particles is along the direction of wave travel (figures 15.1, 15.4) • Example: sound

  6. Wave Characteristics • Amplitude: maximum displacement of any part of a wave from its rest position • Wavelength: minimum distance between particles that have the same displacement and are moving in the same direction • Period: time required for a single wave to pass a given point • Frequency: number of complete waves passing a given point per unit time (Hertz=1/s) • Propagation velocity: velocity of energy transfer of a wave

  7. Transverse Wave  Crest Amplitude  Trough

  8. f = 1 T f = frequency (Hz) T = period (sec) v = velocity (m/s) λ = wavelength (m) T = period f = frequency λ T v = = λf

  9. Superposition of Waves • When two waves pass through the same medium and a new wave is created • Constructive interference: the waves add together to form a larger displacement • Destructive interference: waves add together to form a smaller displacement • Addition is algebraic • See Figure 15.6 • Nodes: areas where waves cancel each other out

  10. Standing Waves • Two waves combine so that there is no propagation of energy along the wave • Wave displacements are constant and motionless • The two waves are of equal amplitude and wavelength (troughs line up with crests) • Musical instruments form standing waves (energy transferred to air producing sound)

  11. Diffraction • Ability of a wave to bend around obstacles in its path • Examples: • Water waves bend around obstacles • Blue sky = scattered light (blue has shortest wavelength & scatters the most) • Red setting sun = scattered light

  12. Electromagnetic Waves                                                                                                                 <> • Transverse wave resulting from a periodic disturbance in an electromagnetic field • Has electric component and magnetic component • Perpendicular to each other • Each perpendicular to direction of travel • Travel at speed of light = c = 3.00 x 108 m/s (substituting: c = λf)

  13. If a tree falls in the woods....? • Three requirements for detection of sound in a physiological sense • Source of sound • Medium for transmitting • Ear to receive • In physical sense, sound is a vibratory disturbance in an elastic medium which may produce the sensation of sound

  14. Sound • Refers to those waves transmitted through a medium with frequencies capable of being detected by the human ear and is produced by a vibrating source • Ringing bell, tuning fork, vocal cords • Will not travel in vacuum • Travels through many media besides air (see Table 15.1)

  15. Speed of Sound • In dry air at STP = 331 m/s • Changes as humidity and temperature change • See equations on page 412

  16. Sound Characteristics • Physical properties • Intensity: energy transferred by sound per unit time through unit area (= power/area) • Frequency • Physiological properties • Loudness: strength of the sensation of sound heard by an observer; describes how strong or faint sensation seems • Pitch: effect of the frequency of sound waves on the ear • Higher pitch = higher frequency • Lower pitch = lower frequency

  17. Doppler Effect • Refers to the variation of the frequency heard when a source of sound and the ear are moving relative to each other • Named after Christian Johann Doppler • When approaching, waves are “pushed together” creating shorter wavelengths and higher frequencies (thus a different sound) • Going away, wavelength increases, frequency decreases (different sound) • See equation on page 415

  18. Resonance • Natural frequency: frequency at which an object vibrates when struck by another object • Resonance/sympathetic vibration: occurs when the natural vibration rates of two objects are the same • Example: Rattles in automobiles, radio receivers, musical instruments

  19. Simple Harmonic Motion • Linear motion in which the acceleration of an object is directly proportional to its displacement from its equilibrium position • Motion is always directed toward the equilibrium position • Displacement: distance from equilibrium • Amplitude: maximum displacement • Period: time required for one vibration • Frequency: number of vibrations per unit time

  20. Pendulum • Suspended object that swings freely back and forth about a pivot • Motion closely approximates simple harmonic motion • Three properties of pendulum (Galileo) • Period is independent of mass • Period independent of amplitude for small arc (<10°) • Can calculate period using equation on pg. 422

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