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Announcements 10/10/12 Prayer Term projects: Proposals due a week from Saturday,
emailed to me with your proposal in body of email. Groups of 2 are encouraged. Groups of 3 with
permission. Just send one email per group, but CC your partners on the email.
Syllabus: “The term project is an opportunity for you to propose and conduct a simple experiment or to theoretically, mathematically, or computationally investigate an aspect of the course in more depth.”
Guidelines: “At least 15 hours of work per person (not including time spent at the hardware store, etc.)” http://www.physics.byu.edu/faculty/colton/courses/phy123resources/project/guidelines.htm
Some ideas to get you thinking: http://www.physics.byu.edu/faculty/colton/courses/phy123resources/project/ideas.htm
Some actual projects: http://www.physics.byu.edu/faculty/colton/courses/phy123resources/project/actualprojects.htm
From warmup Extra time on?
a. First warmup question
Other comments?a. Do we have a strobe light with an adjustable
frequency? They can make a lot of really cool demos for this stuff when combined with standing waves on a string.
b. Can we have answer ranges on the next exam, or at least a comic, or perhaps a comical answer range?
Close toHome
Interference (same wavelength waves)
Path length differencesa. Constructiveb. Destructive
Video: Two outdoor speakers (1:16)
Demo: Moire pattern transparencies
Demo: Hearing test Demo: 2-speaker
interferenceRipple Tankimage: wikipedia
From warmup A wave carries energy. When two waves
destructively interfere, however, their amplitudes cancel out. Where has the energy gone? (Consider waves on a rope for simplicity.)
a. In the moment the waves cancel each other out, the individual bits of the rope are still in motion. The wave energy is 100% transverse kinetic energy at that moment.
From warmup In a standing wave, what is the difference
between the nodes and the antinodes? a. The nodes are the places where
destructive interference continually occurs--the rope looks like it is not moving. The antinodes are the places where constructive interference occurs the most--the rope moves from the max positive displacement to the max negative.
Standing Waves
Will occur whenever you have two waves (same frequency, wavelength) moving in opposite directions
Math: A cos(kx - t) + A cos(kx + t) From trig.: cos(a–b) + cos(a+b) = 2cos(a)cos(b)
Image from Wikipedia
Image by Colton
Standing Waves, cont. Video (on your own time):
http://stokes.byu.edu/teaching_resources/standing_script_flash.html (1:48)
Can occur via reflections of a single wave! But timing (frequency) has to be just right for it to occur over & over again
Demo: rubber tubing Web demo, revisited
http://www.colorado.edu/physics/phet/simulations/stringwave/stringWave.swf
Demo: “Ladies belt”
Standing Wave Patterns Back to rubber tubing demo
a. What kinds of patterns do you get?b. “node” vs “antinode”
Harmonics: allowed frequencies
Standing waves on a string: “Closed-Closed”
For stable patterns: L = _______ What are the frequencies? Relative to the fundamental frequency?
L
L
L
L