ANSWERS
Quiz 12; Problems 3, 4, 5, 9, 10, 11, 25, 26, 39, 40, 42, 43, 49, 50, 51, 52
TURN IN: 4, 10, 26, 40, 42, 50, 52
12/18/2010 08:37:38 PM.  Note correction to problem 4.
4.  time = L/v, where v = speed of sound, which depends on the medium. Sound travels faster in water than in air, so the fish will hear the backfire first.
10.

95 dB = 10 log(2I/Io).
Use the log  property that log(ab) = loga + logb:
95 = 10log2 + 10log(I/Io)
log(I/Io)  = (95 - 10log2)/10.
See example 12-4.

Note this problem is a special case of the following: "If N identical  firecrackers produce a sound level of 95 dB when fired  simultaneously,  what will be the sound level when only one is exploded?"

95 dB = 10 log(NI/Io),  which leads to :

 log(I/Io)  = (95 - 10logN)/10

26.
(a) fn = nv/(4L) , where   v = 343 m/s approximately and n = 1. See figure 12-12 (a).
(b) If L decreases to (2/3)*L, then the resonant frequency in part ( a) (where  v = 343 m/s approximately and n = 1)  will increase  accordingly to a new value.

40.
(a) The difference between the two frequencies is the beat  frequency.  See fig. 12-18.
(b) If each frequency  were reduced by a factor of 4, then divide the difference in part (a) by 4.
42. | f - 350 | = 4 and | f - 355 | = 9.  Use the  basic algebra theorem, that |X| = C  means X = -C or X = C. This means that,

| f - 350 | = 4 can be written as: 
f - 350 = -4 or f - 350 = 4 Hz

and

 | f - 355 | = 9 can be written as: 
f - 355 = -9 or f - 355 = 9 Hz

These translate to:

f  = 346 Hz or f = 354 Hz , for  4 beats /sec

and

f = 346 Hz or f = 364 Hz,   for 9 beats/sec.

Thus the common value of f = 346 Hz.
50. Yo, the listener  is at rest, and the source either moves 

(a) toward you, so the frequency is increased. Use formula 12-12 a.

or 

(b) away from you so the frequency is decreased.  Use formula 12-12 b.

We've been there, done that in class, when I derived the two formulas with vivid diagrams of a source "catching up" with its wave. 
52. 

5.5 Hz = fhigh -  flow  . 

 fhigh = flow /(1 - Vs/V), where Vs = speed of source = 15 m/s and V = speed of sound (about 343 m/s.  Thus, numerically:
5.5 =  flow /(1 - Vs/V) -  flow .  Solve for  flow  . 
12/18/2010 08:37:38 PM.  Note correction to problem 4.
More Hints Later.