A child sits on the ground next to a remote-controlled toy car. At time $$$t=0$$$, the car begins to move in a straight line directly away from the child. The variation with time $$$t$$$ of the velocity of the car along this line is shown in Fig. 4.1. The car's horn continually emits sound of frequency $$$925 \mathrm{~Hz}$$$ between time $$$t=0$$$ and time $$$t=6.0 \mathrm{~s}$$$. The speed of the sound in the air is $$$338 \mathrm{~ms}^{-1}$$$. Determine the frequency, to three significant figures, of the sound heard, by the child, that was emitted from the car horn at time $$$t=3.0 \mathrm{~s}$$$. frequency $$$=$$$ ................................................ $$$\mathrm{Hz}$$$

Physics

IGCSE&ALevel

CAIE

Exam No：9702_w21_qp_22 Year：2021 Question No：4(b)

Answer:

$\begin{aligned} f_{\mathrm{o}} &=f_{\mathrm{s}} v /\left(v+v_{\mathrm{s}}\right) \\ &=925 \times 338 /(338+12) \\ &=893 \mathrm{~Hz} \end{aligned}$

7.3.2 use the expression for the observed frequency when a source of sound waves moves relative to a stationary observer