Physics 235 Winter 2011: Practice Exam #3 Version 2 Solutions
Form #1
Please print your name:_____________________________________________________
Instructions
1.
Fill in your name above
2.
This will be a 1 hour 50 minute, closed book exam.
3.
You may use a calculator, please do not share calculators
4.
For this exam, you can bring three 3x5 cards with any notes you wish to have. You
will also be given all physical constants that you might require to do all the problems.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
1: (10 points) A bat engaged in echolocation emits 30 ms long sound pulses with a
frequency of 75,000 Hz and with an initial intensity of 10
2
W/m
2
. The speed of sound in
air is 340 m/s.
a: (2 points) This sound strikes a moth 22 m away from the bat. What is the intensity of
the sound, measured in decibels, when it strikes the moth?
There is no way to do this without knowing where the initial intensity is
measured. Let’s assume this is very close to the bat, we’ll take 10 cm away. Now
we can work out the intensity at the moth.
2
0
0
0
2
0
2
2
0
0
0
22
0
2
2
2
22
22
22
0
0
22
22
10
10
10
10
12
2
12
2
22
P = 4
4
4
4
4
0.1
10log
10log
20log
100
20log
53.2 dB
10
W/m
10
W/m
22
P
I
r I
r
r I
r
P
I
I
r
r
r
I
r
I
dB
r
b: (3 points) The sound bounces off the moth and returns to the bat. What is the intensity
in decibels of the reflected sound the bat hears?
2
2
2
2
2
moth
0
moth
0
moth 0
return
0
2
2
4
2
2
22
22
22
0
0 moth
22
10
10
10
12
2
2
2
22
P 4
π
r
4
π
r
4
π
r
I
r
r
I
=
=
=
I
r
4
π
r
4
π
r
I
r r
0.01×0.1
dB
=10log
+20log
=100+20log
=
13.7 dB
10
W/m
r
22
c: (2 points)
How much time elapses between the beginning of the bat’s sound pulse and
the return of the echo?
2
2
22 m
65 ms
340 m/s
s
d
t
v
d: (3 points) If the bat is flying toward the moth at a speed of 4 m/s, and the moth is
hovering at rest, what is the frequency of the reflected sound the bat hears?
0
0
0
340
340
4
1.023
76,785 Hz
340
4
340
s
s
b
s
b
s
v
v
v
f
f
f
f
v
v
v
2: (5 points) Two speakers emit sound waves in synch with one another. They are
separated by a distance of 2.0 m. The frequency of the sound they produce is 206 Hz.
Consider a point a distance x from speaker A, along the line between them.
a: (3 points) For what values of x between the speakers will there be constructive
interference between the two?
= 340 m/s / 206 Hz = 1.65 m. The path length difference
PL = (2x) – x = 22x
We want this to be an integer multiple of
, so 22x = n
, which gives
x = 1  n
/2
which has values x = 0.175, 1, 1.825
(for n = 1, 0, and 1)….
b: (2 points) For what values of x will there be destructive interference?
PL = 22x = (2n+1)
/2
x = 1 – (2n+1)
/4
x = 0.5875, 1.4125 (for n = 0 and n = 1)
3: (5 points) Explain why you can see “images” when you look in a mirror but not when
you look in a wall.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Spring '08
 Mckay
 Diffraction, Light, Wavelength

Click to edit the document details