Ch. 13 Waves and Sound

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Ch. 13 Waves and Sound
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Sound

• Tuning forktines vibrate and cause waves in
  surrounding medium.
• Tines vibrating not visible because the tines are
  vibrating at a very high frequency.
• (middle C on the piano keyboard a frequency of
  256 Hz or 256 vibrations per second)

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Sound

• Forced vibration an object is forced to
  vibrate,
• e.g. putting a tuning fork on a tabletable
  vibrates along with tuning fork
• e.g. train moving on railroad trackstracks
  vibrate with train and make it louder

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Natural Frequency and Resonance

• All objects have a natural frequencytend to
  vibrate at a certain frequency
• Resonance forced vibration that matches an
  objects natural frequency

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Examples of Natural Frequency

• e.g. tuning a radio is adjusting natural
  frequency of the electronics of radio to match
  one of the many signals in the air
• e.g. troops marching together across a bridge in
  Manchester, England 1831 matched its natural
  frequency and bridge collapsed

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Sample Problem Skyscraper

• Gusts of wind cause the Sears Building in Chicago
  to sway back and forth, completing a cycle every
  ten seconds. What is
• (a) its frequency and
• (b) its period?

Frequency 1 cycle/10 sec 1/10 Hz Period 10
sec
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Wave Speed

• Does speaking generate wind?
• No, energy of ones voice travels at about 340
  m/s, air itself doesnt travel

Drop a stone in the water and waves move outward
Distance between ripples wavelength
Wave speed frequency x wavelength
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Example Problem Train Cars

• If a train of freight cars, each 10 m long, rolls
  by you at the rate of three cars each second,
  what is the speed of the train?

1 car 1 wave Wavelength 10 m Frequency 3
Hz Speed freq x wavelength 3 Hz x 10 m 30 m/s
10 m x 3 cars 30 m 30 m/1 sec 30 m/s
OR
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Example Problem Ocean

• If a water wave vibrates up and down three times
  each second and the distance between wave crests
  is 2 m, what is
• (a) waves frequency?
• (b) wavelength?
• (c) wave speed?

• 3 cycles/1 second 3 Hz
• 2m
• Wave sp freq x wlength
• 3 cycles/1 s x 2 m 6 m/s

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Waves

• Electromagnetic vibration of electrons within
  atoms on the Sun's surface, travel through the
  vacuum of outer space
• e.g. LIGHT

• Mechanical cannot transmit through a
  vacuum, require a medium in order to transport
  energy
• e.g. SOUND

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Sound and Light

• Similarity Both are vibrations that move through
  space as waves
• Difference
• Soundmovement of vibrations of matter through
  liquids, solids or gases (not through a vacuum)
• Lightvibration of electric and magnetic fields,
  can pass through many materials, can pass through
  a vacuum