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JEE Main 201910 AprilEvening Shift

Exam: JEE Main
Year: 2019
Date: 10 April
Shift: Evening Shift
Subject: Physics
Chapter: Waves and Sound

JEE Main 2019 · 10 April · Evening Shift · Physics · Waves and Sound

A source of sound S is moving with a velocity of 50m s ^ - 1...

A source of sound

S

is moving with a velocity of

50m{s}^{- 1}

towards a stationary observer. The observer measures the frequency of the source as

1000Hz .

What will be the apparent frequency of the source when it is moving away from the observer after crossing him? (Take velocity of sound in air is

350m{s}^{- 1}

)

Options

A $750Hz$
B $857Hz$
C $1143Hz$
D $807Hz$

Correct answer

A. $750Hz$

Step-by-step solution

When source approaches,

{f}_{1} = {f}_{0}\left(\dfrac{v}{v - {v}_{s}}\right)

When source recedes,

{f}_{2} = {f}_{0}\left(\dfrac{v}{v + {v}_{s}}\right)

⇒ \dfrac{{f}_{2}}{{f}_{1}} = \dfrac{v - {v}_{s}}{v + {v}_{s}}

⇒ {f}_{2} = 1000 \times \dfrac{350 - 50}{350 + 50}Hz = 750Hz

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