Announcements 10/13/10

1. Announcements 10/13/10 • Prayer • Change to reading assignment: • For Friday: 18.2 – 18.6 • 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

2. 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) • 1 V • 2 V • 3 V • 2 V • 3 V

3. Plane Waves

4. Spherical Waves

5. 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 • http://stokes.byu.edu/bells.wav

6. The Pie Factory vbelt • Spacing between pies = ? •  = v/f • vs source speed • vo observer speed • v speed of sound (vbelt) • If observermoves toward source (pie maker), she would measure the same ___________ but the pies are coming at her at a faster ________ • If sourcemoves toward observer, the __________ shrinks, but the pie _______ doesn’t change f = vnew/lold f = vold/lnew wavelength speed wavelength speed

7. Doppler, cont. • Stokes Flash video • http://stokes.byu.edu/doppler_script_flash.html • Combine both effects: • What does  mean?

8. 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? • f’ = (1/2) f • f’ = (2/3) f • f’ = (3/2) f • f’ = 2 f • None of the above.

9. 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. • f’ = (1/3) f • f’ = (2/3) f • f’ = f • f’ = 3 f • None of the above.

10. Doppler shift of light • Light source (galaxy) traveling toward you: frequency will _____, wavelength will _____. • See HW 19.4 for equation

11. Astronomy • How far away is a galaxy? Hard question • Edwin Hubble, 1929: Distance away proportional to speed • Distance measured through supernovae observations • 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 • (Yes, it’s OK for LDS to believe in the Big Bang…)

12. 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?

13. θ Sonic Boom Sonic boom manifested by condensation of water in air sinq = vsound/vsource = 1/“Mach number”

14. Sonic Boom Sonic boom of bullet in flight (holographic interferometry?) • How fast is the bullet traveling? • Mach # = 1/sinq

15. Reading Quiz • Which term did not appear in the discussion of wave interference for today’s reading assignment? • antinode • constructive • destructive • path length • superposition

16. Ripple Tank image: wikipedia Interference • Path length • Constructive • Destructive • Video: Two outdoor speakers • Demo: Moire pattern transparencies • Demo: Hearing test • Demo: 2-speaker interference • Speed of sound = ?