Trevino, Denise – Fnmakeup 1 – Due: Jun 4 2005, 11:00 am – Inst: Turner
1
This printout should have 28 questions.
Multiplechoice questions may continue on
the next column or page – fnd all choices
be±ore answering.
The due time is Central
time.
001
(part 1 o± 2) 10 points
A police car is traveling at a speed,
v
c
, to
the le±t. A truck is traveling at a speed,
v
t
, to
the right. The ±requency o± the siren on the
police car is
f
c
.
The speed o± sound in air is
v
a
. Let
v
t
be
the speed o± the
observer
in the truck, and
v
c
be the speed o± the
source
, the police car.
Police
v
c
v
t
Truck
What is the ±requency,
f
t
, heard by an
observer in the moving truck?
1.
f
t
=
v
a

v
t
v
a
+
v
c
f
c
correct
2.
f
t
=
v
a

v
t
v
a

v
c
f
c
3.
f
t
=
v
a
+
v
t
v
a
+
v
c
f
c
4.
f
t
=
v
a
+
v
t
v
a

v
c
f
c
Explanation:
Basic Concepts:
The Doppler shi±ted ±re
quency,
f
0
, heard in the truck is
f
0
=
v
a
±
v
0
v
a
∓
v
s
f ,
(1)
where
v
a
is the speed o± sound in air,
v
o
is the
speel o± the observer, and
v
s
is the speed o±
the source,
The upper sign is used when the relative
velocities are toward oneanother, and
vice
versa
.
Solution:
The relative velocity o± the ob
server is away ±rom the source so the lower
sign is used in the numerator (
± → 
), and
the relative velocity o± the source is away ±rom
the observer so the lower sign is used in the
denominator (
∓ →
+). There±ore Eq. 2 be
comes
f
t
=
v
a

v
t
v
a
+
v
c
f
c
.
This is version two o± ±our versions.
002
(part 2 o± 2) 10 points
A police car is traveling at a speed,
v
c
, to the
le±t. A truck is traveling at a speed,
v
t
, to the
le±t. A wind is blowing in the same direction
as that o± the truck with a speed,
v
w
, to the
le±t. The ±requency o± the siren on the police
car is
f
c
.
The speed o± sound in air is
v
a
.
Police
v
c
v
t
Truck
v
w
wind
What is the ±requency,
f
t
, heard by an
observer in the moving truck?
1.
f
t
=
v
a

v
t
+
v
w
v
a
+
v
c

v
w
f
c
2.
f
t
=
v
a
+
v
t
+
v
w
v
a
+
v
c

v
w
f
c
3.
f
t
=
v
a
+
v
t

v
w
v
a

v
c
+
v
w
f
c
4.
f
t
=
v
a

v
t

v
w
v
a
+
v
c

v
w
f
c
5.
f
t
=
v
a

v
t

v
w
v
a

v
c
+
v
w
f
c
6.
f
t
=
v
a

v
t
+
v
w
v
a

v
c
+
v
w
f
c
7.
f
t
=
v
a
+
v
t

v
w
v
a
+
v
c

v
w
f
c
correct
8.
f
t
=
v
a
+
v
t
+
v
w
v
a

v
c
+
v
w
f
c
Explanation:
The problem must be worked in the ±rame
o± re±erence relative to the air. “
v
t

v
w
” is
the relative velocity o± the truck (observer),
v
o
. “
v
c

v
w
” is the relative velocity o± the car
(source),
v
s
, there±ore
f
t
=
v
a
±
(
v
t

v
w
)
v
a
∓
(
v
c

v
w
)
f
c
.
(2)