First slide

A train is moving with a constant speed u along a circular track. A siren in its engine is emitting a sound of frequency v. If u = v/10, where v is the speed of sound, the apparent frequency of sound as heard by a passenger at the rear end of the train is


Refer to Fig. 

The components of u0andus along the direction of the velocity of sound are u0 cosθ and us cosθ given u0 = us= u. Thus, the source is receding from the observer with a speed u cos θ and the observer is approaching the source with the same speed u cos θ 
 Apparent frequency  v'=vv+ucosθv+ucosθ=v
Hence the correct chose is (d)

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