班級:光電一乙
學號:498L0105
姓名:陳昶宏
98 學年度第 2 學期物理 Chap14 作業
1. A sound wave has a frequency of 700 Hz in air and a wavelength of 0.50 m.
What is the temperature of the air?
v=fλ=700  0.5  350 m
350 m  331 
s
s
273  x
273
x  32.24  C
2. Calculate the sound level in decibels of a sound wave that has an intensity of
4.00 μW/m2.
4 w
m2
求dB
I
4  10 -6
B  10log( )  10log(
)  66.02dB
Io
10 -12
3. At rest, a car’s horn sounds the note A (440 Hz). The horn is sounded while
the car is moving down the street. A bicyclist moving in the same direction
with one-third the car’s speed hears a frequency of 415 Hz. What is the speed
of the car? Is the cyclist ahead of or behind the car?
bike(接近)
car(遠離)
常溫: 25  C  298  K
V= 331 
298
 345 m
s
273
1 

345  v s 



v

v
3
0
f'  f
  v s  32.7 m s
  415  440 
 v  vs 
 345  v s 


聲源(car)遠離觀者(bike)，所以觀者在聲源後方。
4. Two loudspeakers are placed above and below one another, as in Figure 14.12,
and are driven by the same source at a frequency of 500 Hz. (a) What
minimum distance should the top speaker be moved back in order to create
destructive interference between the speakers? (b) If the top speaker is
moved back twice the distance calculated in part (a), will there be
constructive or destructive interference?
r1  x   r2  xr1  r2 
343
 0.686m
500
1
1
最小破壞性干涉  x     0.686  0.343m
2
2
(a) v  f   
r1  2x  - r2  2x
(b)
r1  r2
2x  
波長為偶數倍，所以為建設性干涉
5. A stretched string fixed at each end has a mass of 40.0 g and a length of 8.00
m. The tension in the string is 49.0 N. (a) Determine the positions of the nodes
and antinodes for the third harmonic. (b) What is the vibration frequency for
this harmonic?
(a)
node1:0m
node 2: 8 
1
 2.67 m
3
node 3: 8 
2
 5.3m
3
node 4: 8 1  8m
腹點 1: 5.3 
1
 1.33m
4
腹點 2: 1.33  3  3.99m
腹點 3: 1.33  5  6.65m
(b)