A car is parked 21.0 m directly south of a railroad crossing. A train is approac

Question

A car is parked 21.0 m directly south of a railroad crossing. A train is approaching the crossing from the west, heading directly east at a speed of 50.0 m/s. The train sounds a short blast of its 309-Hz horn when it reaches a point 18.0 m west of the crossing. What frequency does the car's driver hear when the horn blast reaches the car? The speed of sound in air is 343 m/s. (Hint: Assume that only the component of the train's velocity that is directed towards the car affects the frequency heard by the driver.)

Explanation / Answer

the relative velocity of the train to the car when the train is 21m.west of the crossing.
The angle will be 45 degrees, as both the car and train are at that time, 18m. from the crossing.

(cos 45) x 50m/sec = 35.355m/sec. towards the car, at that instant in time.
((343 + 35.355)/343) x 309Hz = 340.85 Hz. perceived by the car's driver

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