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



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Answer 1

Given :

velocity of air , v = 350 m/s

initial frequency , f = 1.5 kHz

velocity of train, vt = 50 m/s

To find :

Frequency observed by the driver after reflection from the wall.

Solution :

First we have to find the frequency observed at the wall. That frequency will be reflected. with the help of the new frequency we can find the observed frequency .

frequency observed at wall , fw = ( v * f ) / ( v - vt )            .........(i)

frequency observed by driver = ( v + vt ) * fw / v              ..........(ii)

putting (i) in (ii) ,

frequency observed by driver = ( v + vt ) * f  / ( v - vt )  

                                                  = ( 400 / 300 ) * 1.5 kHz

                                                  = 2 kHz

The frequency observed by the driver after reflection is 2 kHz .