A train is moving towards a stationary observ | vedclass

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(B) According to the Doppler effect,when a source of sound (train) moves towards a stationary observer,the observed frequency $f$ is given by $f = f_0

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(B) According to the Doppler effect,when a source of sound (train) moves towards a stationary observer,the observed frequency $f$ is given by $f = f_0 \left( \frac{v}{v - v_s} \right)$,where $v$ is the speed of sound and $v_s$ is the speed of the source. This frequency is higher than the source frequency $f_0$ and remains constant as long as the source approaches with constant velocity.When the train passes the observer and starts moving away,the observed frequency becomes $f = f_0 \left( \frac{v}{v + v_s} \right)$,which is lower than the source frequency $f_0$ and remains constant as long as the source moves away with constant velocity.Therefore,the frequency $f$ will be higher than $f_0$ initially and will suddenly drop to a value lower than $f_0$ at the moment the train passes the observer. This behavior is correctly represented by graph $B$.

$A$ train is moving towards a stationary observer. Which of the following curves best represents the frequency $f$ received by the observer as a function of time $t$?

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