According to Doppler effect there is a change in frequency or wavelength of a wave in relation to observer who is moving relative to the wave source, which is given as f′=(
v
v−vs
)f where f ′ is the apparent frequency; f is frequency of source; vS is velocity of source; v is velocity of sound Given
,f=340Hz;v=340ms−1;vs=10ms−1
Therefore,
f′=(
340
340−10
)×340=350.3030Hz∼350Hz
Thus, frequency of whistle heard by a stationary observer on the platform is 350 Hz