Ch
apter 11 Answer to Problems 3 ,13, 21 , 25 , 49
3.The intensity of the sound wave from a jet airplane as it is taking off is 1.0
2
W/m
2
at a distance
of 5.0 m. What is the intensity of the sound wave that reaches the ears of a person standing at a
distance of 120 m from the runway? Assume that the sound wave radiates from the airplane equally in
all directions.
Answer: The intensity of the sound as a function
of the distance is given by
I
2
/I
1
= (P/4pr
2
2
) / (P/4pr
1
2
) = r
1
2
/r
2
2
I
2
= I
1
x(r
1
2
/r
2
2
) = (1.0x10
2
W/m
2
)x(5.0m/120m)
2
=170 x10
3
W/m
2
13. What is the wavelength of a wave whose speed and period are 75.0m/s and 5.00ms respectively
Given: v=75.0m/s ; T=5.00ms
Question:
?
T=5.00ms= 5.00x10
3
sec
f=1/T = (1/5.00x10
3
) Hz;
v=
f
=v/f = (75.0m/s )(5.00x10
3
)=0.375m
21.
. A wave on a string has equation
y
(
x, t
) = (4.0 mm) sin (
t
–
kx
)
where
= 6.0
2
rad/s and
k
= 6.0 rad/m. (a) What is the amplitude of the wave? (b) What is the
wavelength? (c) What is the period? (d) What is the wave speed? (e) In which direction does the wave
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 Spring '09
 HUANG
 Hertz, Wavelength, sound wave

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