Two ambulances, \(\text{A}\) and \(\text{B}\), are travelling along a straight road, both with the same constant velocity, \(v\). Both ambulances have their sirens on and the sounds produced are identical and have a constant frequency.

Ambulance \(\text{A}\) is travelling directly towards a stationary observer, while ambulance \(\text{B}\) is travelling directly away from the stationary observer, as shown in the diagram below.
 

Which one of the following best describes the frequency of each siren as measured by the stationary observer, compared to the frequency the observer would measure if the ambulances were stationary?

1. The observer measures each siren's frequency to be lower.
2. The observer measures each siren's frequency to be higher.
3. The observer measures the frequency of ambulance \(\text{A}\)'s siren to be lower and the frequency of ambulance \(\text{B}\)'s siren to be higher.
4. The observer measures the frequency of ambulance \(\text{A}\)'s siren to be higher and the frequency of ambulance \(\text{B}\)'s siren to be lower.

Lee listens while a police car with a loud siren comes towards her, travels past her and then continues on away from her.

Compared with the sound she would hear from the siren if the police car were stationary, the sound has.

1. a higher frequency as the car comes towards her and a lower frequency when the car moves away.
2. a lower frequency as the car comes towards her and a higher frequency when the car moves away.
3. a lower intensity as the car comes towards her and a greater intensity when the car moves away.
4. the same frequency at all times.

Alex hears the siren from a stationary fire engine.

Compared with the sound Alex hears from the stationary fire engine, the sound Alex will hear as the fire engine approaches him will have increased

1. speed.
2. period.
3. amplitude.
4. frequency.

A distant fire truck travelling at 20 ms\(^{-1}\) to a fire has its siren emitting sound at a constant frequency of 500 Hz.

Chris is standing on the edge of the road. Assume that the fire truck is travelling directly towards him as it approaches and directly away from him as it goes past. The arrangement is shown in Figure 14.
 

1. On the diagram below, sketch the frequency that Chris will hear as the truck moves towards him and then moves away from him. The \(500 \text{ Hz}\) siren signal is shown as a dotted line for reference. No calculations are required.  (2 marks)
    

2. Name the physics principle involved in Chris’s experience.  (1 mark)
   

Which one of the following statements best describes an observation of the Doppler effect for sound?

1. a decrease in frequency received when a source of sound moves towards you
2. a decrease in frequency received when moving towards a stationary source of sound
3. an increase in frequency received when moving towards a stationary source of sound
4. a decrease in wavelength received when moving away from a stationary source of sound