You are moving at 120 km/h toward a stationary train.The train blows its 0.400-kHz whistle.Take the speed of sound to be 340 m/s.What frequency do you hear?

Question

Quizplus · Accepted Answer

The formula for the apparent frequency heard by a moving observer is given by:
f' = f (v +/- vr) / (v +/- vs)
Where:
f = frequency emitted by the source
v = speed of sound
vr = speed of observer towards/away from the source
vs = speed of source towards/away from the observer

Here, the observer (you) is moving towards the stationary train, so vr = 120 km/h = 33.3 m/s towards the train. The train is stationary, so vs = 0 m/s. The frequency of the whistle is f = 0.400 kHz = 400 Hz, and the speed of sound is v = 340 m/s.

Substituting these values in the formula:
f' = 400 (340 + 33.3) / (340)
f' = 439 Hz

Therefore, the frequency you hear is 439 Hz, which is given by choice (B).

You Are Moving at 120 Km/h Toward a Stationary Train