# When a source passes a stantionary observer. The apparent frequency decreases by 5%. Find the speed of the source.  The speed of the sound is 340m/s

llltkl | College Teacher | (Level 3) Valedictorian

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The apparent change in frequency of sound due to motion of source and observer relative to each other is known as Doppler effect. The frequency heard by the observer is apparent frequency. It is higher than the real frequency if they approach each other and lower, if they move away from each other.

Here, the observed frequency is 5% lower, meaning the source is moving away from the stationary observer.

So apparent frequency `hatn = n* v/(v+v_s)`

Where, n is the real frequency, `v_s` is the velocity of the source and v, the velocity of sound in the medium.

Putting the values in equation,

`95 = 100* 340/(340+v_s)`

`rArr`  `(340+v_s)= (340*100)/95`

`rArr`  `v_s = (340*100)/95-340`

= `17.89` m/s

= `(17.89*3600)/1000` km/hr.

= 64.4 km/hr.

Therefore, the speed of the moving sound source was 17.89m/s or, 64.4 km/hr.

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