A whistle of frequency is blown on a train that moves straight towards a vertical…

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

Problem

A whistle of frequency

400\,\text{Hz}

is blown on a train that moves straight towards a vertical cliff with speed

15\,\text{m\,s}^{-1}

. The speed of sound in air is

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

. The sound reflected by the cliff returns to the whistle blower. What beat frequency does the blower hear between the direct sound and the echo? (a)

0\,\text{Hz}

(b)

37\,\text{Hz}

(c)

18\,\text{Hz}

(d)

35\,\text{Hz}