A whistle of frequency is stationary

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

Problem

A whistle of frequency

500\,\text{Hz}

is stationary, while another identical whistle moves directly towards a stationary observer with speed

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

. If the speed of sound in air is

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

, how many beats per second will the observer hear? (a) 10 (b) 5 (c) 9 (d) 8