Are there any traveling circus left?

Physics help- a police car is traveling at a speed vc to the left...?

• A police car is traveling at a speed vc to the left and a truck is traveling at a speed vt to the left. A wind is blowing in the same direction as that of the truck with a speed vw to the left. The frequency of the siren on the police car is fc. What is the frequency ft heard by an observer in the moving truck? The speed of sound in air is va. 1. ft= (va+vt+vw)/(va+vc-vw)*fc 2. ft= (va-vt-vw)/(va+vc-vw)*fc 3. ft= (va+vt-vw)/(va+vc-vw)*fc 4. ft= (va-vt-vw)/(va-vc+vw)*fc 5. ft= (va-vt+vw)/(va+vc-vw)*fc 6. ft= (va+vt+vw)/(va-vc+vw)*fc 7. ft= (va+vt-vw)/(va-vc+vw)*fc 8. ft= (va-vt+vw)/(va-vc+vw)*fc Thank you!

• Answer:

  Doppler effect: f = Fo•(V + Vr)/(V+Vs) Vr < 0 if receiver is moving away from sender Vs < 0 if sender is moving toward receiver So we're down to choices 4 and 8 (the only ones with -ve signs for vt and vc). Then consider that "the speed of sound in air is va". If the air is moving at vw, then I think the speed of sound is "va + vw" so I would pick #8, which adds "vw" in both cases. I am surprised by this, as it is not at all intuitive that windspeed will affect the Doppler effect.