A loudspeaker in a parked car is producing sound whose frequency is 20,535 Hz. A

Question

A loudspeaker in a parked car is producing sound whose frequency is 20,535 Hz. A healthy young person with normal hearing is standing nearby on the sidewalk but cannot hear the sound because the frequency is too high. When the car is moving, however, this person can hear the sound. (Normal hearing means a person will hear sounds from 20 Hz to 20 kHz.)

(a) Is the car moving toward or away from the person? moving away from the person moving toward the person Why? moving toward increases the wavelength and decreases the frequency moving toward decreases the wavelength and increases the frequency moving away decreases the wavelength and increases the frequency moving away increases the wavelength and decreases the frequency

(b) If the speed of sound is 343 m/s, what is the minimum speed of the moving car? m/s

Explanation / Answer

a)

the car moving away from the person

moving away increases the wavelength and decreases the frequency

b)

from doppler effect of sound

f = fo*v/(v+vs)

fo = 20535 Hz

f = 20000 Hz

v = speed of sound = 343 m/s

20000 = 20535*343/(343 + vs)

vs = 9.17 m/s <<<-------answer

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