The siren of a police car, which is travelling at a constant speed along a straight horizontal road, emits sound waves of constant frequency - NSC Physical Sciences - Question 6 - 2019 - Paper 1

The graphs show that the time intervals between the sound waves detected by Q are longer compared to those detected by P. This indicates that the sound waves are being stretched as the source (the police car) moves away, confirming that the police car is indeed moving away from detector Q.

To calculate the frequency () of the sound waves detected by detector P, we can use the formula:

$f = \frac{1}{T}$

where T is the period of the wave. From graph A, we can see that the period T is approximately 17 imes 10^{-4} s. Thus:

$f = \frac{1}{17 \times 10^{-4}} \approx 588.24 \text{ Hz}$

To find the speed (v) of the police car, we can use the formula:

$v = f \lambda$

where  is the frequency and $\\lambda$ is the wavelength. From graph B, the distance between two consecutive peaks can be measured to approximate the wavelength, which is found to be 0.578 m. Thus:

$v = 588.24 \times 0.578 \approx 340 \text{ m s}^{-1}$

This confirms that the speed calculated matches the known speed of sound in air.