Announcements 10/12/11 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

Logarithm Review Log10(x) is the inverse of 10y

→ if x = 10y then y = log10(x)

a. I.e. “10 to the what equals 22?” answer: 1.3424 calculator:

log10(22) Review of “Laws of Logs”:

– 1. log(ab) = log(a) + log(b)– 2. log(an) = n log(a)

log10(100) = ? Translation: 10 to what equals 100?

ln(100) = ? (“ln” = loge = log2.71828…)

Translation: e to what number =100? (4.605…)

Ambiguity: “log(100)”…could be either log10 or ln

Question: log10(1,000,000) = ? Question: If log(3) = 0.477, what is log(300)?

Thought Question

A +3 dB increase is just about a factor of 2 in intensity. How many dB represents a factor of 4 increase in intensity?

a. +4b. +6c. +8d. +9e. +10

What if you need to solve for I?

Other Power and Intensity Scales

Power or Intensity sound

a. dB β = 10 log(I/I0) I0 = 10-12 W/m2

microwaves/rfa. dBm β = 10 log(P/P0) P0 = 1 mW

electronics/electrical circuitsa. dB β = 10 log(P2/P1) (ratio only)

Thought Question

How much power is 30 dBm?a. 1 mWb. 3 mWc. 30 mWd. 100 mWe. 1000 mW

dBm: β = 10 log(P/P0) P0 = 1 mW

Thought Question

How much power is 33 dBm?a. 1003 mWb. 1006 mWc. 1100 mWd. 2000 mWe. 3000 mW

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, sound, and light waves as well as waves on a string)

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

Doppler Effect Demo: Doppler Speaker 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

Stokes Come, Come, Ye Saints recording

a. http://stokes.byu.edu/bells.wav (0:32)

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

ne

w=

(vb

elt-

v s)/

f s

Doppler, cont.

Combine both effects:

What does mean?

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

(1:50)

v v

v vo

s

f f

Distant light sourcea. Traveling toward youb. Traveling away from

you See HW 19.4 for

equation

Astronomy Edwin Hubble, 1929: Distance to galaxies is

proportional to their speeda. Distance measured through Cepheid variable

star observations, “standard candle”b. 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 distance to far away objects… 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_script_flash.html (2:47) http://stokes.byu.edu/boomray_script_flash.html

(2:53)

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

Interference Path length

a. Constructiveb. Destructive

Video: Two outdoor speakers (1:16)

Demo: Moire pattern transparencies

Demo: Hearing test Demo: 2-speaker

interferenceRipple Tankimage: wikipedia