1. The problem statement, all variables and given/known data A truck moving at 36 m/s passes a police car moving at 45 m/s in the opposite direction. If the frequency of the siren relative to the police car is 500 Hz, what is the frequency heard by an observer in the truck after the police car passes the truck? (The speed of sound in air is 343 m/s.) a)396 Hz b) 636 Hz c)361 Hz d)393 Hz e)617 Hz 2. Relevant equations f′ = [(v + vO)/(v − vS)]f 3. The attempt at a solution Using the equation above, as they are moving away from each other, i made vO = -36 m/s and vS = -45m/s. Plugging in those values, I get 396 Hz, which is option a) however the correct answer appears to be 636 Hz, which can only be obtained assuming that they are moving towards each other.