By referring to the direction in which energy is transferred, state what is meant by a longitudinal wave.
A vehicle moves at constant speed around a broad circular track. It keeps sounding its horn, which produces a single note of frequency $1.2\,\text{kHz}$. A distant observer is far from the track, as shown in Fig. 4.1. Fig. 4.2 shows how the frequency $f$ of the horn sound detected by the observer changes with time. The time for the vehicle to complete one lap of the track is $T$.
Explain why the frequency of the sound detected by the observer is sometimes higher than and sometimes lower than $1.2\,\text{kHz}$.
State the name of the phenomenon in (b)(i).
On Fig. 4.1, mark with a letter X the position of the vehicle when it emitted the sound detected at time $T$.
On Fig. 4.1, mark with a letter Y the position of the vehicle when it emitted the sound detected at time $\frac{9T}{4}$.
The speed of sound in air is $320\,\text{m s}^{-1}$. Use Fig. 4.2 to find the speed of the vehicle in (b).