question_answer

A railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer beside the railway track. Which of the following graphs best represent the variation of frequency of the sound n heard by the observer with the time t?

A)   

B)  

C)  

D)  

Correct Answer: B

Solution :

 \[\upsilon =\]velocity of sound. \[{{f}_{1}}=\frac{\upsilon }{\upsilon -u}f\] When source moves towards stationary observer. \[f=\frac{\upsilon }{\upsilon +u}f\] When source moves away from observer. \[\therefore \] \[{{f}_{1}}<f\] f does not depend on time elapse (t).