15 Traveling Waves and Sound

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• 15 Traveling Waves and Sound

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Types of Waves
A transverse wave
A longitudinal wave
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Waves on Strings and in Air
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Snapshot Graphs
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Constructing a History Graph
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Checking Understanding
The graph below shows a snapshot graph of a wave
on a string that is moving to the right. A point
on the string is noted. Which of the choices is
the history graph for the subsequent motion of
this point?
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Answer
The graph below shows a snapshot graph of a wave
on a string that is moving to the right. A point
on the string is noted. Which of the choices is
the history graph for the subsequent motion of
this point?
(b)
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Checking Understanding
The graph below shows a history graph of the
motion of one point on a string as a wave moves
by to the right. Which of the choices is the
correct snapshot graph for the motion of the
string?
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Answer
The graph below shows a history graph of the
motion of one point on a string as a wave moves
by to the right. Which of the choices is the
correct snapshot graph for the motion of the
string?
(d)
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Conceptual Example Problems
A wave travels back and forth on a guitar string
this is responsible for making the sound of the
guitar, as we will see. As the temperature rises,
the tension in a guitar string decreases. How
does this change the speed of a wave on the
string? How do you measure the temperature of a
flame if the temperature is higher than a probe
can handle? One possible solution is to use
sound. A source emits a pulse of sound on one
side of the flame, which is then measured by a
microphone on the other side. A measurement of
the time between the emission and the reception
of the pulse allows a determination of the
temperature. Explain how this technique works.
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Example Problem

• A particular species of spider spins a web with
  silk threads of density 1300 kg/m3 and diameter
  3.0 µm. A typical tension in the radial threads
  of such a web is 7.0 mN. If a fly lands in this
  web, which will reach the spider first, the sound
  or the wave on the web silk?

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Sinusoidal Waves
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Checking Understanding

• For this sinusoidal wave
• What is the amplitude?
• 0.5 m
• 1 m
• 2 m
• 4 m

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Answer

• For this sinusoidal wave
• What is the amplitude?
• 0.5 m
• 1 m
• 2 m
• 4 m

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Checking Understanding

• For this sinusoidal wave
• What is the wavelength?
• 0.5 m
• 1 m
• 2 m
• 4 m

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Answer

• For this sinusoidal wave
• What is the wavelength?
• 0.5 m
• 1 m
• 2 m
• 4 m

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Checking Understanding

• For this sinusoidal wave
• What is the frequency?
• 50 Hz
• 100 Hz
• 200 Hz
• 400 Hz

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Answer

• For this sinusoidal wave
• What is the frequency?
• 50 Hz
• 100 Hz
• 200 Hz
• 400 Hz

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Example Problems
The new generation of cordless phones use radio
waves at a frequency of 5.8 GHz. What is the
wavelength of these radio waves? A speaker emits
a tone of a particular frequency. Suppose the air
temperature increases. What happens to the
wavelength of the sound?
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Example Problem
The water in the open ocean is in constant
motion, carrying long-wavelength waves moving at
relatively high speeds. Under steady winds, the
amplitude of these waves can get quite large.
Suppose a boat is at rest in the open ocean. The
wind has created a steady wave with wavelength
190 m traveling at 14 m/s. (In fact, the ocean
will support a mix of waves, but for steady winds
of 30-40 knots, this is the most prevalent
wavelength, and the correct speed for a wave of
this wavelength in deep water.) The top of the
crests of the waves is 2.0 m above the bottom of
the troughs. (This wave height is quite typical
for windy days in the Atlantic Ocean. The
Southern Ocean, with its planet-circling stretch
of open water, supports much larger waveswave
heights of 7 m are quite common.) What is the
maximum vertical speed of the boat as it bobs up
and down on the passing wave? What is the maximum
vertical acceleration?
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Example Problem
Lets use the data from the previous problem
again. Suppose the boat is sailing at 6.0 m/s in
the same direction as the motion of the waves. At
t ? 0 s the boat is at the bottom of a trough.
How high above this lowest point will the boat be
at t ? 10 s?
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Sound and Light Waves
The speed of sound varies with the medium. Light
and other electromagnetic waves in vacuum and in
air move at the same speed, 3.00 x 108 m/s.
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