A student is sitting on the beach, observing a power boat moving at speed on the sea. The boat has a siren emitting a constant sound of frequency 420 Hz. The boat moves around in a circular path with a speed of 25 m s⁻¹. The student notices that the pitch of the siren changes with a regular pattern.

a. Explain why the pitch of the siren changes, as observed by the student.

b. Determine the maximum and minimum frequencies that the student will hear.

c. At which point in the boat’s motion will the student hear the most high-pitched note?
(Speed of sound in air = 330 m s⁻¹.)