A truck is traveling at 74.5 kilometers per hour away from you. The driver is blowing the horn which has a frequency of 415 Hz. The speed of the sound is 346 m/s. What is the observed frequency of the sound?

fo = 335 Hz let fo = frequency heard

f = actual frequency

v = speed of sound

vs = speed of source

For the first one,

fo = fv / (v - vs)

74.5 km/h = 20.69 m/s

vs = - 20.69m/s since it is moving away from you

fo = (415Hz) (346m/s) / (346m/s - (-20.69m/s))

fo = 392 Hz

For the second one,

fo = fv / (v - vs)

82.8 km/h = 23 m/s

vs = 23m/s

fo = (312Hz) (331m/s) / (331m/s - (23m/s))