Answer:
The actual frequency produced by the train whistle is 444 Hz.
Step-by-step explanation:
Given;
speed of the train, Vs = 120 km/h = 33.33 m/s
observed frequency, Fo = 0.4 kHz = 400 Hz
speed of sound in air, V = 340 m/s
Note: as the train moves away, the observed frequency will be smaller than the source frequency. Thus, the source frequency which is frequency of the train whistle will be greater than 400 Hz.
The source frequency is given by Doppler effect formula;
![F_s = F_o [(V)/(V-V_s) ]\\\\F_s = 400[(340)/(340-33.33) ]\\\\F_s = 400(1.109)\\\\F_s = 443.6 \ Hz](https://img.qammunity.org/2021/formulas/physics/college/awbmocmlhs8fw6yr0xnwri1t0gihb2806o.png)
Fs = 444 Hz
Therefore, the actual frequency produced by the train whistle is 444 Hz.