the siren of a fire engine that is driving northward at 30.0 m/s emits a frequency of 2000 Hz. A truck in front of this fire engine is moving northward at 20.0 m/s.
a. what frequency does the truck driver hear?
b. the siren's sound reflects off the back of the truck and is heard by the driver of the fire engine. What is this reflected frequency heard by the fire engine's drive?