A source of sound emits sound waves at frequency f0. It is moving… | JEE Main 2015 (11 Apr, Shift 1)

Q66JEE Main 2015 · 11 APR · SHIFT 1Difficult

Doppler Effect

A source of sound emits sound waves at frequency f0. It is moving towards an observer with fixed speed $v$ s ( $v$ s < $v$ , where $v$ is the speed of the sound in air). If the observer were to move towards the source with speed $v$ 0, one of the following two graphs (A and B) will give the correct variation of the frequency f heard by the observer as $v$ 0 is changed.

The variation of f with $v$ 0 is given correctly by:

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Answer: (a)

Both approaching: $f = f_0\frac{v + v_0}{v - v_s} = \frac{f_0 v}{v - v_s} + \frac{f_0}{v - v_s}v_0$
$f$ is linear in $v_0$ with a positive intercept, which is graph A.
Slope $= \frac{f_0}{v - v_s}$
So the answer is (a).

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Doppler Effect question from JEE Main 2015 (11 Apr, Shift 1)

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