Adv Physics

1
Adv Physics

• Chapter 14
• Sections 1,2,5

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Longitudinal Wave

• Motion of material is parallel to direction
  disturbance/energy travels
• Compression region in a longitudinal wave where
  the density and pressure is greater than normal
• Rarefaction region in a longitudinal wave where
  the density and pressure is less than normal

3
How is sound produced?

• Sound is produced by a vibrating object
• The frequency of the source is the same as the
  frequency of the sound produced
• Once a sound wave is created its frequency can
  NEVER change

4
Sound Waves

• Audible sound waves with frequencies in the
  range of human hearing (20 20000 Hz)
• Infrasonic sound waves with frequencies below
  the range of human hearing
• Ultrasonic sound waves with frequencies above
  the range of human hearing
• By age 70 most people cant hear above 8000 Hz

5
Pitch

• perceived highness or lowness of a sound
• - determined by frequency
• http//edu.larc.nasa.gov/connect/machine.swf
• - higher the frequency the higher the pitch
• - subjective

6
Loudness

• Volume
• - determined by energy wave carries
• http//edu.larc.nasa.gov/connect/machine.swf
• - larger the amplitude, the louder it
• sounds

7
Speed of Sound

• Depends on medium and temperature
• Medium in solids particles are closer together
  so easier and quicker to pass on disturbance
• Temperature as material warms up the particles
  collide more frequently so disturbance is passed
  on more quickly

8
Speed of Sound

• v in air at 0 degrees C is 331 m/s
• (740 mi/h)
• v increases by 0.6 m/s for every 1 degree
  Celsius increase in temperature
• In a solid the change in speed is less dramatic
  with temperature because the particles are
  already very close

9
Doppler Effect

• Change in pitch or frequency detected by an
  observer due to the motion of the source and/or
  the receiver
• http//www.ecocardiografia.info/Doppler_effect.swf
  

10
Doppler Effect
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Source Toward Stationary Observer

• Observer will receive a higher frequency and
  higher pitch if car is approaching
• Observer will receive a lower frequency and lower
  pitch if car is leaving

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Doppler Effect

• How does the speed of the source effect your
  results?
• http//www.upscale.utoronto.ca/GeneralInterest/Har
  rison/Flash/ClassMechanics/DopplerWaveFronts/Doppl
  erWaveFronts.swf
• The faster the source moves, the greater the
  change in frequency

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Source Toward Stationary Observer

• f f / (1 vs/v)
• where f - perceived frequency
• f - actual frequency
• vs - speed of source
• v - speed of sound
• Note faster source moves the greater the change
  in frequency

14
Source Moving Away From Stationary Observer

• f f / (1 vs/v)
• where f - perceived frequency
• f - actual frequency
• vs - speed of source
• v - speed of sound

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Sample Problem

• A high speed train is traveling at 44.7 m/s
  when the engineer sounds the 415 Hz warning horn.
  The speed of sound is 343 m/s. What are the
  frequency and wavelength of the sound as
  perceived by a person at the crossing who the
  train is approaching? Leaving?

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Observer Toward Stationary Source

• f f (1 vo/v)
• where f - perceived frequency
• f - actual frequency
• vo - speed of observer
• v - speed of sound

17
Observer Away From Stationary Source

• f f (1 - vo/v)
• where f - perceived frequency
• f - actual frequency
• vo - speed of observer
• v - speed of sound

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Both Moving

• - If moving toward each other
• f f (1 vo/v) / (1 vs/v)
• - If moving away from each other
• f f (1 - vo/v) / (1 vs/v)

19
Doppler Effect with Light

• Red shift decrease in perceived frequency of
  light due to source and observer moving apart
• Blue shift increase in perceived frequency of
  light due to source and observer moving toward
  each other

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Sample Problem

• An ambulance travels down a highway at a speed
  of 33.5 m/s. Its siren emits a sound at a
  frequency of 400 Hz. What is the frequency heard
  by a passenger in a car traveling at a speed of
  24.6 m/s in the opposite direction as the car
  approaches the ambulance and as the car moves
  away from the ambulance?

22
Sample Problem

• Standing at a crosswalk, you hear a frequency of
  560 Hz from the siren on an approaching police
  car. After the police car passes, the observed
  frequency of the siren is 480 Hz. Determine the
  cars speed from these observations.