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

PHYSICS · Single correct · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics

Problem

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