A truck is traveling 70.0 kph away from you. The driver is blowing the horn, which has a frequency of 400 Hz. The air temperature is 25°C. What is the observed frequency of the horn?

Question

aaronq · Answer

speed of truck =70 km/h*$\sf \dfrac{1~hr}{60~min}*\dfrac{1~min}{60~sec}*\dfrac{1000~m}{1~km}$= 19.44 m/s

speed of sound in air =343 m/s

doppler formula for receding object stationary observer

$\sf f_{obs}=\dfrac{f_o*v}{v_s+v}=\dfrac{400~Hz*343~m/s}{19.44m/s+343~m/s}=378.54=379~Hz$