A fire engine is driving directly away from a solid wall at 35 m/s on a hot 30.0

Question

A fire engine is driving directly away from a solid wall at 35 m/s on a hot 30.0 C day while its siren emits a steady 455. Hz sound. The speed of sound on this day is 349 m/s. Compute the frequency heard by a stationary listener standing near the wall. Compute the wavelength of the waves as they approach the stationary listener. A firefighter on the back of the fire engine hears the sound waves after they are reflected from the wall. What is the frequency of this reflected wave as heard by this firefighter?

Explanation / Answer

There is a formula in your text. The fire engine is moving through a stationary medium. So for A) the stationary listener sees a departing fire engine and hears a lower frequency, as per the formula. For B), the lower frequency has a certain wavelength, as per the relationship between frequency, wavelength and speed of sound (in a stationary medium) For C) the reflected sound waves have the same lower frequency as in A) and chase a departing fire engine. Twice the effect. Think of the distance for the round trip, engine -> wall -> engine, which increases at twice the speed of the fire engine.

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