Announcements 10/13/10
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Prayer
Change to reading assignment:
a. For Friday: 18.2 – 18.6
b. 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:
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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 V
b. 2 V
c. 3 V
d. 2 V
e. 3 V
Plane Waves
Spherical Waves
Doppler Effect
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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 Factory
vbelt
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Spacing between pies = ?
a.  = v/f
vs source speed
f = vnew/old f = vold/new
vo observer speed
v speed of sound (vbelt)
If observer moves toward source (pie maker), she
wavelength but the pies are
would measure the same ___________
speed
coming at her at a faster ________
wavelength
If source moves toward observer, the __________
speed doesn’t change
shrinks, but the pie _______
Doppler, cont.
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Stokes Flash video
a. http://stokes.byu.edu/doppler_script_flash.html
Combine both effects:
v  vo
f f
v  vs
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What does  mean?
Thought Question
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v  vo
f f
v  vs
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) f
b. f’ = (2/3) f
c. f’ = (3/2) f
d. f’ = 2 f
e. None of the above.
Thought Question
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v  vo
f f
v  vs
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) f
b. f’ = (2/3) f
c. f’ = f
d. f’ = 3 f
e. None of the above.
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Light source (galaxy)
traveling toward you:
frequency will _____,
wavelength will _____.
See HW 19.4 for equation
Astronomy
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How far away is a galaxy? Hard question
Edwin Hubble, 1929: Distance away proportional to
speed
a. Distance measured through supernovae observations
b. How did he measure speed?
– Doppler shift of spectral lines!
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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
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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?
Sonic Boom
Sonic boom
manifested by
condensation
of water in air
θ
sinq = vsound/vsource = 1/“Mach number”
Sonic Boom
Sonic boom of
bullet in flight
(holographic
interferometry?)
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How fast is the bullet traveling?
a. Mach # = 1/sinq
Reading Quiz
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Which term did not appear in the discussion of
wave interference for today’s reading
assignment?
a. antinode
b. constructive
c. destructive
d. path length
e. superposition
Interference
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Path length
a. Constructive
b. Destructive
Video: Two outdoor speakers
Demo: Moire pattern
transparencies
Demo: Hearing test
Demo: 2-speaker interference
a. Speed of sound = ?
Ripple Tank
image: wikipedia