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# A sound source of frequency 400 Hz is moving towards an observer with a velocity of 50 m/s while the observer is moving away from the source with a velocity of 40 m/s . Find the frequency heard by the observer . (Velocity of sound is 340 m/s )

$(a)\;423.7\;Hz\qquad(b)\;391.6\;Hz\qquad(c)\;413.8\;Hz\qquad(d)\;405.7\;Hz$

Answer : $\;391.6\;Hz$
Explanation :
By Doppler's effect
$f^{|}=(\large\frac{v-v_{0}}{v-v_{s}})\;f$
$v-v_{0}\;$ as observer is moving away
$f^{|}=(\large\frac{340-40}{340-50})\;400$
$=\large\frac{300}{290}\;400$
$=413.8\;Hz\;.$
edited Mar 13, 2014 by yamini.v