An observer is standing on the platform of a railway station. A train goes through the station without stopping. If the frequency of the train whistle decreases by a factor of 1.2 as it approaches and then passes him, calculate the speed of the train assume the speed of sound in air is 343 m/s).
A) 112 km/hr
B) 31.2 km/hr
C) 56.0 km/hr
D) 62.0 km/hr
E) 122 km/hr

Question

Quizplus · Accepted Answer

The Doppler effect formula for frequency change is used: f' = f(v_sound / (v_sound ± v_source)). Given the frequency decreases by a factor of 1.2, solve for v_source (train speed) using v_sound = 343 m/s. The correct speed is approximately 112 km/hr.

An Observer Is Standing on the Platform of a Railway