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Class 11 NEET Physics Waves Questions Questions

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Question : 57 of 96

Marks: +1, -0

A car is moving towards a high cliff. The driver sounds a horn of frequency

f

. The reflected sound heard by the driver has frequency

2 f \cdot

v

is the velocity of sound, then the velocity of the car, in the same velocity units, will be

$\frac{v}{\sqrt{2}}$
$\frac{v}{3}$
$\frac{v}{4}$
$\frac{v}{2}$

Solution:

1^{\text{st}}

the car is the source and at the cliff, one observes

f'

.

\Rightarrow f' = \frac{v}{v-v_{s}} f

2^{\text{nd}}

cliff is now source. It emits frequency

f'

and the observer is now the driver who observes

f''

.

\Rightarrow f'' = \left[ \frac{v+v_{o}}{v} \right] f'

or

2 f = \left[ \frac{v+v_{o}}{v-v_{s}} \right] f

\Rightarrow 2 v - 2 v_{o} = v + v_{o}

[ as

v_{s} = v_{o}

\Rightarrow v_{o} = \frac{v}{3}

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