Question

An engine driver moving towards a wall with velocity of 50m/s emits a note of frequcency1.5kHz. The frequency of note after reflection from the wall as hear by the engine driver when speed of sound in air is 350m/s is

Answer 1

Answer:

Hence, the required frequency of note = 2 KHz

Explanation:

The reflected sound appears to propagate opposite to that of moving engine. Thus, the source and observer can be presumed to approach each other with same velocity.

The Doppler effect is observed whenever the source of waves is moving with respect to an observer.

So according to Doppler effect,

v' = [ v(v + Vo) ] / (v - Vs)

=> v' = v [(v + Vo) / (v - Vs)] { Vo = Vs }

=> v' = 1.5 [ (350 + 50) / (350 - 50) ]

=> v' = 1.5 [ 400 / 300 ]

=> v' = 400 / 200 = 2 KHz

Hence, the required frequency of note = 2 KHz