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.

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