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
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