Answer :
The relationship between the frequency heard by the observer in motion and the original frequency of the sound is given by (Doppler effect)
[tex]f'= \frac{v}{v-v_s}f [/tex]
where
f' is the frequency heard by the observer
v is the speed of the wave (the speed of sound)
[tex]v_s[/tex] is the speed of the source relative to the observer (= the speed of the car), and it is negative when the source is approaching the observer
f is the original frequency of the sound
By re-arranging the formula, we get
[tex]v_s=v( 1- \frac{f'}{f}) [/tex]
and by plugging the data of the problem into the equation, we find
[tex]v_s = (343 m/s)( 1- \frac{470 Hz}{450 Hz})=-15.2 m/s [/tex]
so, the car is approaching the observer at 15.2 m/s.
[tex]f'= \frac{v}{v-v_s}f [/tex]
where
f' is the frequency heard by the observer
v is the speed of the wave (the speed of sound)
[tex]v_s[/tex] is the speed of the source relative to the observer (= the speed of the car), and it is negative when the source is approaching the observer
f is the original frequency of the sound
By re-arranging the formula, we get
[tex]v_s=v( 1- \frac{f'}{f}) [/tex]
and by plugging the data of the problem into the equation, we find
[tex]v_s = (343 m/s)( 1- \frac{470 Hz}{450 Hz})=-15.2 m/s [/tex]
so, the car is approaching the observer at 15.2 m/s.