At an instant, what is the perceived frequency that you hear and how fast is it

Question

At an instant, what is the perceived frequency that you hear and how fast is it changing? if a sound with frequency f is produced by a source traveling along a line with speed v and an observer is traling with speed v along the same line from the opposite direction toward the source, then the frequency of the sound heard by the observer is where c is the speed of sound, about 332 m/s. (this is the Doppler effect) Suppose that, at a particular moment, you are in a train traveling at 34 m/s and accelerating at 1.2 m/s^2. A train is appraoching you from the opposite direction on the other track at 40 m/s, accelerating at 1.4 m/s^2, and sounds its whistle, which has a frequency of 460 Hz. At that instant, what is the perceived frequency that you hear and how fast is it changing? We are also given f0 = (c+V0/c-Vs)fs

Explanation / Answer

At that instant we don't hear anything. It takes some time for the sound of the approachingtrainto reach us. You don't tell us how far off the approaching train is at the instant mentioned so, in view of the accelerations, we cannot calculate what the frequency will be at the somewhat later instant when the sound does reach us.

EDIT: Your additional detail doesn't address the lack of the missing distance. approaching means "getting closer", which implies "not yet arrived".

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