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JEE Main Physics Class 11 Waves Part 1 Questions
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© examsnet.com
Question : 25
Total: 100
The driver of a bus approaching a big wall notices that the frequency of his bus’s horn changes from 420 Hz to 490 Hz when he hears it after it gets reflected from the wall. Find the speed of the bus if speed of the sound is 330 ms
–
1
.
[Sep. 04, 2020 (II)]
91 kmh
–
1
81 kmh
–
1
61 kmh
–
1
71 kmh
–
1
Validate
Solution:
From the Doppler's effect of sound, frequency appeared at wall
f
w
=
330
330
−
v
.
f
........(i)
Here, v = speed of bus,
f = actual frequency of source
Frequency heard after reflection from wall (f') is
f
′
=
330
+
v
330
.
f
w
=
330
+
v
330
−
v
.
f
⇒
490
=
330
+
v
330
−
v
.
420
⇒
v
=
330
×
7
91
≈
25.38
m
∕
s
=
91
k
m
∕
s
© examsnet.com
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