A child sits on a rotating horizontal platform in a playground. The child moves with a constant speed along a circular path, as illustrated in [Figure 4.1]. An observer is standing a long distance away from the child. During one particular revolution, the child, moving at a speed of 7.5 m s⁻¹, starts blowing a whistle at point P and stops blowing it at point Q on the circular path. The whistle emits sound of frequency 950 Hz. The speed of sound in air is 330 m s⁻¹.