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JEE Main 201910 JanuaryMorning Shift

Exam: JEE Main
Year: 2019
Date: 10 January
Shift: Morning Shift
Subject: Physics
Chapter: Waves and Sound

JEE Main 2019 · 10 January · Morning Shift · Physics · Waves and Sound

A train moves towards a stationary observer with speed 34m / s ....

A train moves towards a stationary observer with speed

34m / s .

The train sounds a whistle and its frequency registered by the observer is

{f}_{1} .

If the speed of the train is reduced to

17m / s,

the frequency registered is

{f}_{2} .

If speed of sound is

340m / s,

then the ratio

{f}_{1} / {f}_{2}

is:

Options

A $21 / 20$
B $20 / 19$
C $19 / 18$
D $18 / 17$

Correct answer

C. $19 / 18$

Step-by-step solution

For stationary observers, $f = \left(\dfrac{v}{v - u}\right) \times \left(originalfrequency\right)$

{f}_{1} = \left(\dfrac{340}{340 - 34}\right)f

{f}_{2} = \left(\dfrac{340}{340 - 17}\right)f

\dfrac{{f}_{1}}{{f}_{2}} = \left(\dfrac{340 - 17}{340 - 34}\right) = \dfrac{19}{18}

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