Physics (AS & A Level) · Oct / Nov 2019 · Paper 13 · Question 24

A siren emits sound of frequency 1000 Hz. The siren moves at 20 m s⁻¹ towards an observer who is standing still. The speed of sound in the air is 330 m s⁻¹. Which expression would correctly give the frequency heard by the observer?

1. A\frac{1000 \times 330}{330 + 20}
2. B\frac{1000 \times 330}{330 - 20}
3. C\frac{1000(330 + 20)}{330}
4. D\frac{1000(330 - 20)}{330}

As printed in the paper