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FOR SCIENTISTS AND ENGINEERS
physics
a strategic approach
THIRD EDITION
randall d. knight
© 2013 Pearson Education, Inc.
Chapter 20 Lecture
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© 2013 Pearson Education, Inc.
Chapter 20 Traveling Waves
Chapter Goal: To learn the basic properties of traveling waves.
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Chapter 20 Preview
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Chapter 20 Preview
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Chapter 20 Preview
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Chapter 20 Preview
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Chapter 20 Preview
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© 2013 Pearson Education, Inc.
Chapter 20 Preview
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© 2013 Pearson Education, Inc.
Chapter 20 Reading Quiz
Slide 20-9
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A graph showing wave displacement versus position at a specific instant of time is called a
Snapshot graph.
History graph.
Bar graph.
Line graph.
Composite graph.
Reading Question 20.1
Slide 20-10
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Answer: A
© 2013 Pearson Education, Inc.
A graph showing wave displacement versus position at a specific instant of time is called a
Snapshot graph.
History graph.
Bar graph.
Line graph.
Composite graph.
Reading Question 20.1
Slide 20-11
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Answer: A
© 2013 Pearson Education, Inc.
A graph showing wave displacement versus time at a specific point in space is called a
Snapshot graph.
History graph.
Bar graph.
Line graph.
Composite graph.
Reading Question 20.2
Slide 20-12
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Answer: B
© 2013 Pearson Education, Inc.
A graph showing wave displacement versus time at a specific point in space is called a
Snapshot graph.
History graph.
Bar graph.
Line graph.
Composite graph.
Reading Question 20.2
Slide 20-13
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Answer: B
© 2013 Pearson Education, Inc.
A wave front diagram shows
The wavelengths of a wave.
The crests of a wave.
How the wave looks as it moves toward you.
The forces acting on a string that’s under tension.
Wave front diagrams were not discussed in this chapter.
Reading Question 20.3
Slide 20-14
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Answer: B
© 2013 Pearson Education, Inc.
A wave front diagram shows
The wavelengths of a wave.
The crests of a wave.
How the wave looks as it moves toward you.
The forces acting on a string that’s under tension.
Wave front diagrams were not discussed in this chapter.
Reading Question 20.3
Slide 20-15
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Answer: B
© 2013 Pearson Education, Inc.
The constant, k, introduced in Section 20.3 on Sinusoidal Waves is
The Boltzman’s constant, with units: J/K.
The Coulomb constant, with units: N m2/c2.
The force constant, with units: n/m.
The wave number, with units: rad/m.
The wavelength, with units: m.
Reading Question 20.4
Slide 20-16
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Answer: D
© 2013 Pearson Education, Inc.
The constant, k, introduced in Section 20.3 on Sinusoidal Waves is
The Boltzman’s constant, with units: J/K.
The Coulomb constant, with units: N m2/c2.
The force constant, with units: n/m.
The wave number, with units: rad/m.
The wavelength, with units: m.
Reading Question 20.4
Slide 20-17
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Answer: D
© 2013 Pearson Education, Inc.
The waves analyzed in this chapter are
String waves.
Sound and light waves.
Sound and water waves.
String, sound, and light waves.
String, water, sound, and light waves.
Reading Question 20.5
Slide 20-18
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Answer: D
© 2013 Pearson Education, Inc.
The waves analyzed in this chapter are
String waves.
Sound and light waves.
Sound and water waves.
String, sound, and light waves.
String, water, sound, and light waves.
Reading Question 20.5
Slide 20-19
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Answer: D
© 2013 Pearson Education, Inc.
Chapter 20 Content, Examples, and QuickCheck Questions
Slide 20-20
© 2013 Pearson Education, Inc.
The Wave Model
The wave model is built around the idea of a traveling wave, which is an organized disturbance traveling with a well-defined wave speed.
The medium of a mechanical wave is the substance through or along which the wave moves.
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© 2013 Pearson Education, Inc.
A Transverse Wave
A transverse wave is a wave in which the displacement is perpendicular to the direction in which the wave travels.
For example, a wave travels along a string in a horizontal direction while the particles that make up the string oscillate vertically.
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© 2013 Pearson Education, Inc.
A Longitudinal Wave
In a longitudinal wave, the particles in the medium move parallel to the direction in which the wave travels.
Here we see a chain of masses connected by springs.
If you give the first mass in the chain a sharp
push, a disturbance travels down the chain by compressing and expanding the springs.
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© 2013 Pearson Education, Inc.
Wave Speed
The speed of transverse waves on a string stretched with tension Ts is:
Where is the string’s mass-to-length ratio, also called the linear density:
Slide 20-24
© 2013 Pearson Education, Inc.
These two wave pulses travel along the same stretched string, one after the other. Which is true?
QuickCheck 20.1
vA > vB
vB > vA
vA = vB
Not enough information to tell.
Slide 20-25
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© 2013 Pearson Education, Inc.
These two wave pulses travel along the same stretched string, one after the other. Which is true?
QuickCheck 20.1
vA > vB
vB > vA
vA = vB
Not enough information to tell.
Slide 20-26
Wave speed depends on the properties of the medium, not on the amplitude of the wave.
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© 2013 Pearson Education, Inc.
For a wave pulse on a string to travel twice as fast, the string tension must be
QuickCheck 20.2
Increased by a factor of 4.
Increased by a factor of 2.
Decreased to one half its initial value.
Decreased to one fourth its initial value.
Not possible. The pulse speed is always the same.
Slide 20-27
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© 2013 Pearson Education, Inc.
For a wave pulse on a string to travel twice as fast, the string tension must be
QuickCheck 20.2
Increased by a factor of 4.
Increased by a factor of 2.
Decreased to one half its initial value.
Decreased to one fourth its initial value.
Not possible. The pulse speed is always the same.
Slide 20-28
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© 2013 Pearson Education, Inc.
Snapshot Graph
A graph that shows the wave’s displacement as a function of position at a single instant of time is called a snapshot graph.
For a wave on a string, a snapshot graph is literally a picture of the wave at this instant.
Slide 20-29
© 2013 Pearson Education, Inc.
One-Dimensional Waves
The figure shows a sequence of snapshot graphs as a wave pulse moves.
These are like successive frames from a movie.
Notice that the wave pulse moves forward distance
x = vt during the time interval t.
That is, the wave moves with constant speed.
Slide 20-30
© 2013 Pearson Education, Inc.
History Graph
A graph that shows the wave’s displacement as a function of time at a single position in space is called a history graph.
This graph tells the history of that particular point in the medium.
Note that for a wave moving from left to right, the shape of the history graph is reversed compared to the snapshot graph.
Slide 20-31
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An Alternative Look at a Traveling Wave
Slide 20-32
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Example 20.2 Finding a History Graph From a Snapshot Graph
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Example 20.2 Finding a History Graph From a Snapshot Graph
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© 2013 Pearson Education, Inc.
Slide 20-35
Example 20.2 Finding a History Graph From a Snapshot Graph
© 2013 Pearson Education, Inc.
This is a snapshot graph at
t = 1 s of a wave pulse traveling to the right at 1 m/s. Which graph below shows the wave pulse at t = –1 s?
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