Announcements 10/14/11

Prayer Lab 3 ends Saturday

a. See email for some more info about the “uncertainty principle” used in the lab

Interference Path length

a. Constructiveb. Destructive

Video: Two outdoor speakers (1:16)

Ripple Tankimage: wikipedia

Reading Quiz In a standing wave, the points that have

the maximum vibration are called:a. nodesb. antinodes

Standing Waves

Demo: Rubber tubingImage from Wikipedia

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)

Can occur via refletions of a single wave! But timing (frequency) has to be just right for it to occur over & over again

Web demo, revisited http://www.colorado.edu/physics/phet/simulations/stringwave/stringWave.swf

Image by Colton

Demos Video:

http://stokes.byu.edu/standing_script_flash.html (1:48)

Demo: “Ladies belt”

Videos: Bowling ball (0:50), goblet (1:42) Youtube (on your own time): Tacoma Narrows bridge Demo: Trumpet

“Resonance”

Standing Wave Patterns Back to Ladies belt demo

a. What kinds of patterns do you get?b. “node” vs “antinode”

Harmonics

Standing waves on a string: “Closed-Closed”

For stable patterns: L = _______ What are the frequencies? Relative to the fundamental frequency?

L

L

L

L

Standing waves in air: “Open-Open”

Quick writing: draw what you think the second and third harmonics should look like

For stable patterns: L = _______ What are the frequencies? Relative to the fundamental frequency? Demo: Trumpet, again

Open end Open end

L

Example: organ pipe

Standing waves in air: “Open-Closed”

Quick writing: draw what you think the second, third, and fourth harmonics should look like

For stable patterns: L = _______ What are the frequencies? Relative to the fundamental frequency? Why wasn’t trumpet in this category?

Open end Closed end

L

Standing Waves in 2D

Again, from Wikipedia

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Standing Waves in 3D From Colton and Wienkes, Rev. Sci. Instr., 2009

a. Light waves, frequency = 10 GHz (microwaves)

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Do they exist? Class poll