Physics

Problem

A stationary observer hears a fire-engine siren at a frequency of

770\,\text{Hz}

. The rest frequency of the siren is

700\,\text{Hz}

, and the speed of sound in air is

340\,\text{m}\!\cdot\!\text{s}^{-1}

. Assuming the engine moves directly along the line joining it and the observer, what is the magnitude of the engine’s speed? a)

34\,\text{m}\!\cdot\!\text{s}^{-1}

toward the observer b)

31\,\text{m}\!\cdot\!\text{s}^{-1}

away from the observer c)

31\,\text{m}\!\cdot\!\text{s}^{-1}

toward the observer d)

24\,\text{m}\!\cdot\!\text{s}^{-1}

toward the observer