Physics 102 Homework #3
Due Noon, Tuesday, February 12, 2008
Submit your paper in the bin in the hall adjacent to Park 354.
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Reading: Knight, Jones, & Field § 16
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Problems: Four problems will be chosen for grading. They will be selected at random from
among those problems indicated with asterisks. You do not need to submit solutions to the
non-asterisk problems.
“KJF” means your textbook, College Physics, by Knight, Jones, and Field. The problems
in this homework are deliberately presented in a different order than given in the textbook.
For problems involving sound in air, use a speed of 343 m/s unless some other speed is
specified. For problems involving light, radio waves, etc., use a speed of 3.0 × 108 m/s.
1. KJF Problem16.1
2. KJF Problem 16.9
3. * KJF Problem 16.10
4. * KJF Problem 16.12
5. * KJF Problem 16.16. Add part (c): draw a picture of the standing wave described
in the first two sentences of this problem. You might want to do this part first.
6. * The world’s largest pipe organ is the Wanamaker Organ in the Wanamaker store
Hecht’s store Lord & Taylor store Macy’s store in Center City Philadelphia. Be sure
to see it while you’re living in the area. It consists of more than 28,000 organ pipes,
the longest of which is 32 feet long and the shortest of which 1/4 inch long. What are
the fundamental frequencies of the longest and shortest pipes? Assume the longest
pipe is open on both ends, and that the shortest pipe is open on one end and closed
on the other end. For English-Metric conversion factors, consult the inside front cover
of your text, or use the internet (Google: “32 feet in meters”).
7. KJF Problem 16.43
8. KJF Problem 16.49
9. * When you walk with a cup of coffee (diameter of cup 8 cm), at a pace of about 1
step per second, the coffee builds up the “sloshing” until eventually, after a few steps,
it starts to spill over the top of the cup. What does this have to do with waves and
resonance? What is the speed of waves in the coffee?
10. KJF Problem 16.52
11. KJF Problem 16.35
12. KJF Problem 16.71
13. KJF Problem 16.59
Continued on the next page....
14. * The figure below shows two speakers, A and B, which are emitting identical,
synchronized sound waves at 830 Hz. A person standing at point C hears a relatively
loud sound. She then starts walking toward point D, and she notices that the sound
gets quieter, reaching a minimum when she arrives at D. The distance from speaker
A to D is 2.50 m. The distance from speaker B to D is 2.30 m. It is a cold day, and
the speed of sound is not 343 m/s.
(a) From the information given above, what is the wavelength of the sound?
(b) From the information given above, what is the speed of the sound?
(c) She remains at point D, and someone slowly increases the frequency of the sound
emitted by the two speakers. It starts to sound louder to her. What is the first
frequency at which the sound intensity goes through a maximum?
A
C
B
D
15. * The sound produced by the speaker in the drawing below has a frequency of
12 000 Hz and arrives at the microphone via two different paths. The sound waves
simultaneously travel through the left tube, sxm, which has a fixed length, and the
right tube, sym, the length of which can be changed by moving the sliding section
(similar to a trombone). The tubes are filled with a gas, but it is not air at standard
pressure and temperature.
The sound waves traveling via the two paths combine at the microphone and interfere.
As the length of path sym is changed, the sound loudness detected by the microphone
changes. When the sliding section is pulled out by 0.020 m the loudness changes from
a maximum to a minimum. Based on this measurement, what is the speed of sound
in the gas in the tube?
Hint #1: first find the wavelength of the sound, λ, then use it to calculate v.
Hint #2: think carefully about how the distance traveled by the sound wave traveling
through the sliding section changes when it is pulled out by 0.020 m. This is an easy
place to make a factor-of-2 error.
speaker
S
sliding section
X
Y
M
microphone