A siren on the top of the police car moving with a speed of 30 m/s emits sound at frequency of 823Hz. Ahead of the car, there is a truck moving in the same direction as the car but with a speed of 20 m/s. Assume the speed of sound in air to be 340m/s.
a) What frequency does the driver of the truck hears as the police car approaches it?
b) What frequency does the driver of the truck hears after the police car has passed it and is receding from it?