Physics 101: Lecture 32 Waves and Sound

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Physics 101 Lecture 32Waves and Sound

• Todays lecture will cover Textbook Sections 16.1
  - 16.5
• Review Simple Harmonic Motion (Chapter 10)

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Review Simple Harmonic Motion

• The position x of an object moving in simple
  harmonic motion as a function of time has the
  following form
• x A cos (wt)
• i.e. the object periodically moves back and forth
  between
• the amplitudes xA and xA.
• The time it takes for the object to make one full
  cycle is
• the period T2p/w1/f, where f is the frequency
  of the
• motion.
• Thus, the angular speed in terms of T and f reads
• w 2p/T and w 2 p f

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What is a wave ?

• Nature of waves
• A wave is a traveling disturbance that transports
  energy from place to place.
• There are two basic types of waves transverse
  and longitudinal.
• Transverse the disturbance travels
  perpendicular to the
• direction of travel of the wave.
• Longitudinal the disturbance occurs parallel to
  the line of
• travel of the wave.
• Examples
• Longitudinal Sound waves (e.g. air moves back
  forth)
• Transverse Light waves (electromagnetic waves,
  i.e. electric and magnetic disturbances)
• The source of the wave, i.e. the disturbance,
  moves continuously in
• simple harmonic motion, generating an entire
  wave, where each
• part of the wave also performs a simple harmonic
  motion.

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Types of Waves
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Wave Properties
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Wave Properties...
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Wave Properties...
v ? / T

• The speed of a wave is a constant that depends
  only on the medium,
• not on the amplitude, wavelength or period
• ? and T are related !
• ? v T or ? 2? v / ??????????????(sinc
  e?T 2? / ??? or ? v / f
  (since T 1/ f ) ?????????????
• Recall f cycles/sec or revolutions/sec
• ? 2?f

Is the speed of a wave particle the same as the
speed of the wave ? No. Wave particle performs
simple harmonic motion vA w sin wt.
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Concept Question

• Suppose a periodic wave moves through some
  medium. If the period of the wave is increased,
  what happens to the wavelength of the wave
  assuming the speed of the wave remains the same?
• 1. The wavelength increases
• 2. The wavelength remains the same
• 3. The wavelength decreases

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Concept Question

• The speed of sound in air is a bit over 300 m/s,
  and the speed of light in air is about
  300,000,000 m/s. Suppose we make a sound wave and
  a light wave that both have a wavelength of 3
  meters. What is the ratio of the frequency of the
  light wave to that of the sound wave?
• 1. About 1,000,000.
• 2. About 1,000.
• 3. About 0.000001.

f v/? fL/fS vL/vS 1,000,000
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Transverse Waves on a String (e.g. Guitar)
Speed
TTension the greater the tension in the string
the greater the pulling force
the particles exert on each other
and the faster the wave travels. mmass/length
of the string m/L linear density of the
spring the smaller the mass the greater the
acceleration for the same pulling force and the
faster the wave travels.
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Concept Question

• A rope of mass M and length L hangs from the
  ceiling with nothing attached to the bottom (see
  picture). Suppose you start a transverse wave at
  the bottom end of the rope by jiggling it a bit.
  As this wave travels up the rope its speed will
• 1. Increase
• 2. Decrease
• 3. Stay the same

the tension gets greater as you go up
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Happy Thanksgiving !