Question

a truck travelling along at straight road sounds its horn, which has a frequency of 500HZ. if an observer besides the road behind the truck measures the frequency to be 480Hz ,how fast is the truck moving?assume the speed of sound in air to be 337 m/s

Answer 1

the frequency heard is lower so the truck is moving away from your microphone.
c = 337 m/s
https://formulas.tutorvista.com/physics/doppler-shift-formula.html
480 = 337 (500) / [337 + v]
337 + v = 337 (500/480)
337 + v = 351.04
v = 14.04 m/s

Answer 2

Fo = (Vs - Vo)/(Vs + Vt) * Fh = 480.
(337-0)/(337 + Vt) * 500 = 480,
Vt = 14.0 m/s = Velocity of the truck.

Fo = Freq. heard by observer.