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 u→0 and u→s 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=vHence the correct chose is (d)