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Question

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 the speed of the sound is 330 ms1

A
91 km h1
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B
81 km h1
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C
61 km h1
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D
71 km h1
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Solution

The correct option is A 91 km h1
From the Doppler's effect of sound, frequency appeared/heard at the wall is,

fw=330330v.f .....(i)

Here,
v speed of bus,
f actual frequency of the source,

The frequency (f) heard by the driver of the bus, after reflection from the wall is,

f=330+v330.fw=330+v330v.f

490=330+v330v×420

v=330×791=25.38 m/s

=25.38×18591 km/hr

Hence, (A) is the correct answer.

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