Physics

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}

37\,\text{Hz}

18\,\text{Hz}

35\,\text{Hz}