whiteboard work 1.a child on a playground swing makes a complete to-and-fro swing every 2 seconds....
TRANSCRIPT
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Whiteboard Work
1. A child on a playground swing makes a complete to-and-fro swing every 2 seconds.
a. What is the frequency of the swinging?
b. What is the period of the swinging?
Don’t forget the units!
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Waves
Part 1: Basics
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Objectives
• Identify the parts of a wave.
• Relate wave speed, frequency, wavelength, and period.
• Trace the movement of the medium in a string wave, in a sound wave, and in a compression wave in a spring.
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What’s the Point?
• How are waves described?
• Why do waves occur?
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Week Outline
• Waves in One Dimension– Features of Waves– Types of Waves
• Combinations of Waves– Interference– Standing Waves
• Waves in Two dimensions– Ocean waves
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Waves and Vibrations
• Vibrations: Repeat in time– no net displacement of disturbance– no net displacement of medium
• Waves: Repeat in time and space– wave travels– no net displacement of medium
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• Wavelength: crest-crest distance• Trough: low point
• Period: crest-crest-timing
Features of a Wave
• Crest: high point
crest
trough
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Features of a Wave• Amplitude A: (crest height – trough height) / 2
A
u
• Frequency f: repeats in a given time (= 1/T)
• Velocity u: speed of crest motion
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Relations between Features
• Period T = 1 / f; Frequency f = 1 / T
• Velocity u = / T
• Wavelength = uT = u / f
• Frequency f = u / ; Period T = / u
= f
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Group Poll Question
Doubling the frequency of a wave while keeping its speed constant will cause its wavelength to
A. increase.B. decrease.C. stay the same.
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Group Poll Question
Doubling the speed of a wave while keeping its wavelength constant will cause its frequency to
A. increase.B. decrease.C. stay the same.
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Group Poll Question
Doubling the wavelength of a wave while keeping its frequency constant will cause its speed to
A. increase.B. decrease.C. stay the same.
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Group Whiteboard Work
2. A wave generator produces 10 pulses each second. The pulses travel at 300 cm/s.
a. What is the period of the waves?
b. What is the wavelength of the waves?
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Wave Pulse in a Rope
• Why does the pulse move?
• What determines its speed?
• What is happening inside the rope?
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Prediction Poll
Increasing the tension on a rope will make waves travel along the rope
A. faster.B. slower.C. At the same speed (no effect).
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Prediction Poll
Increasing the length density (mass per meter) of a rope will make waves travel along the rope
A. faster.B. slower.C. At the same speed (no effect).
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Points to Ponder
The particles of the string change their motion as the wave travels. What force accelerates them?
a cb
d
What are the velocity and acceleration of the string particles at the following positions? Why?
a. middle (leading edge)b. crestc. middle (trailing edge)d. trough
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Types of Waves
• Motion of the medium is perpendicular to the direction the wave travels: transverse wave (example: string wave)
• Motion of the medium is parallel to the direction the wave travels: longitudinal wave (examples: sound wave, slinky wave)
• Animation
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Poll Question
“The Wave” performed by sports fans is an example ofA. a longitudinal wave.B. a transverse wave.C. an unnatural wave.
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Combining Waves
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Group Work
0
–3
3
3. Sketch the wave resulting from the addition of the two waves shown at one instant.
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Group Work
3. Sketch the wave resulting from the addition of the two waves shown at one instant.
result0
–3
3
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Interference
• Constructive: Sum of waves has increased amplitude
• Destructive: Sum of waves has decreased amplitude
• Two-wave simulation
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Beats
• Waves of similar frequency combine to give alternating times of constructive and destructive interference
• Distinctive “waa-waa” sound with beat frequency equal to the difference in frequency of the component waves (Why?)
• (sound files)
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Reading for Next Time
• Sound
• Big ideas:– What sound is– How sound is created– What affects the way sound travels