as observer travelling with a constant velocity of 20m/s passes close to a stationary source of sound and noticed that there is a change of frequency of 50Hz as he passes the source. What is the frequency of the source? (speed of sound in air=340m/s)

Answer 1

Fo = (Vs-Vo)/(Vs+Vg) * Fg = Fg-50.
(340-20)/(340+0) * Fg = Fg-50,
(320/340)* Fg = Fg - 50,
32Fg = 34Fg - 1700,
Fg = 850 Hz. = Frequency of generator or stationary source.

Answer 2

Fo = Freq. of observer.
Vs = Velocity of sound.
Vg = Velocity of generator or stationary source.
Vo = Velocity of observer.