(a)[2]
State the meaning of the Doppler effect.
(b)
A child is seated on a rotating horizontal platform in a playground. The child travels at constant speed along a circular path, as shown in Fig. 4.1. A distant observer stands far from the child. During one complete turn, the child, moving at $7.5\,\text{m s}^{-1}$, begins blowing a whistle at point $P$ and stops blowing it at point $Q$ on the circular path. The whistle produces sound with frequency $950\,\text{Hz}$. The speed of sound in air is $330\,\text{m s}^{-1}$.
(b(i))[2]
Determine the highest frequency of the sound heard by the distant observer.
(b(ii))[2]
Describe how the frequency of the sound heard by the distant observer changes.