A source of sound of frequency `1000 H_(Z)` moves unifornly along a straight line with velocity `0.8` times velocity of sound . An observer is located at a distance `l= 250 m` from this line. Find
(a) the frequency of the sound at instant when the source is closest to the observer.
(b) the distance of the source when he observer no change in the frequency.

Options

A. 2130.8Hz
B. 2425.7Hz
C. 2777.8Hz
D. 3000.0Hz

Answer

A

Explanation

`V_(S)=0.8V:f_(0)=1000Hz`.

Time taken by wave pulse to go from A to O is same as the time taken by source to move from A to it’s nearest distance position from observer (here point B)
Let this time be 't'
From `DeltaAOB`
`costheta=(0.8vxxt)/(vxxt)=0.8`
`f=f_(0)((V+V_(0))/(V-V_(S)))`
`f=1000((V)/(V-V_(S)costheta))=1000((V)/(V-0.8xx0.8))=1000((1)/(1-0.64))=2777.8Hz`