Announcements 10/13/10 Prayer Change to reading assignment:

a. For Friday: 18.2 – 18.6b. For Monday: 18.7 and PpP 4.1

Exams graded, returned to boxes. See email. Tutorial Lab will be a bit understaffed this Friday and

Saturday because many TAs will be at a conference. A student asked me if you could ever have a situation

where the group velocity and phase velocity were in different directions. I found out you can very easily create animated gifs with Mathematica, so here is his answer:

http://www.physics.byu.edu/faculty/colton/courses/phy123-fall10/weirdwave.gif

Thought question:

You have a 1 volt amplitude sine wave. You want to go up 3 dB in power. How many volts do you need? (Recall: Power ~ amplitude2; true for voltages as well as waves on a string)

a. 1 Vb. 2 Vc. 3 Vd. 2 Ve. 3 V

Plane WavesPlane Waves

Spherical WavesSpherical Waves

Doppler Effect What happens if the source is moving? What happens if the observer is moving? Key point:

Frequency is _______________when the source and observer approach each other, ______________ when they go away from each other

Demo: Doppler Speaker Stokes Come, Come, Ye Saints recording

a. http://stokes.byu.edu/bells.wav

The Pie Factoryvbelt

Spacing between pies = ? a. = v/f

vs source speed

vo observer speed

v speed of sound (vbelt) If observer moves toward source (pie maker), she

would measure the same ___________ but the pies are coming at her at a faster ________

If source moves toward observer, the __________ shrinks, but the pie _______ doesn’t change

wavelengthspeed

f = vnew/old

wavelengthspeed

f = vold/new

Doppler, cont.

Stokes Flash videoa. http://stokes.byu.edu/doppler_script_flash.html

Combine both effects:

What does mean?

v v

v vo

s

f f

Thought Question

On a particularly windy day the wind is blowing at half the speed of sound. If I am standing still and a cow is flying towards me at the same velocity as the wind, by what factor will I hear his mooing shifted in frequency?

a. f’ = (1/2) fb. f’ = (2/3) fc. f’ = (3/2) fd. f’ = 2 fe. None of the above.

v v

v vo

s

f f

Thought Question

If I am standing still and a cow is also standing still, by what factor will I hear the frequency his mooing shifted by if the wind is blowing at half the speed of sound in the direction from the cow to me.

a. f’ = (1/3) fb. f’ = (2/3) fc. f’ = fd. f’ = 3 fe. None of the above.

v v

v vo

s

f f

Light source (galaxy) traveling toward you: frequency will _____, wavelength will _____.

See HW 19.4 for equation

Astronomy How far away is a galaxy? Hard question Edwin Hubble, 1929: Distance away proportional

to speeda. Distance measured through supernovae

observationsb. How did he measure speed?

– Doppler shift of spectral lines! That’s now a standard technique for today’s

astronomers when they want to measure distances… just measure Doppler shift.

Hubble’s Law and the Big Bang a. (Yes, it’s OK for LDS to believe in the Big

Bang…)

Sonic Boom

http://stokes.byu.edu/boom_flash.html http://stokes.byu.edu/boomray_script_flash.html

Happens with all types of waves whenever the sources is traveling faster than the speed of the wave…

…so, what is “sonic boom” of water waves?

θ

Sonic boom manifested by condensation of water in air

Sonic Boom

sin = vsound/vsource = 1/“Mach number”

Sonic boom of bullet in flight(holographic interferometry?)

Sonic Boom

How fast is the bullet traveling?a. Mach # = 1/sin

Reading Quiz Which term did not appear in the discussion

of wave interference for today’s reading assignment?

a. antinode

b. constructivec. destructived. path lengthe. superposition

Interference Path length

a. Constructiveb. Destructive

Video: Two outdoor speakers

Demo: Moire pattern transparencies

Demo: Hearing test Demo: 2-speaker

interferencea. Speed of sound = ?

Ripple Tankimage: wikipedia