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JEE Mains 05-Sep-2020 Shift 2 Solved Paper
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© examsnet.com
Question : 8
Total: 75
A driver in a car, approaching a vertical wall notices that the frequency of his car horn, has changed from
440
H
z
to
480
H
z
,
when it gets reflected from the wall. If the speed of sound in air is
345
m
∕
s
,
then the speed of the car is
36
k
m
∕
h
r
24
k
m
∕
h
r
18
k
m
∕
h
r
54
k
m
∕
h
r
Validate
Solution:
f
1
=
frequency heard by wall
=
f
s
=
(
v
s
v
s
−
v
c
)
f
2
=
frequency heard by driver after reflection from wall
f
2
=
(
v
s
+
v
c
v
s
)
f
1
=
(
v
s
+
v
c
v
s
−
v
c
)
f
0
f
2
f
0
=
v
s
−
v
c
v
s
+
v
c
48
44
=
v
s
−
v
c
v
s
+
v
c
12
(
v
s
+
v
c
)
=
11
(
v
s
−
v
c
)
23
v
c
=
v
s
v
c
=
v
s
23
=
345
23
=
15
m
∕
s
=
15
×
18
5
=
54
k
m
∕
h
r
© examsnet.com
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