Physics 9702 · AS & A Level · Doppler effect for sound waves

Doppler effect for sound waves — practice question

A child is seated on the ground beside a remote-controlled car. When $t = 0$, the car starts to travel in a straight line directly away from the child. Figure 4.1 shows how the car’s velocity along this line varies with time $t$. Between $t = 0$ and $t = 6.0\,\text{s}$, the car’s horn continuously produces sound of frequency $925\,\text{Hz}$. The speed of sound in air is $338\,\text{m s}^{-1}$.
(a(i))[1]

Describe qualitatively the change, if any, in the frequency of the sound heard by the child that comes from the car horn, from time $t = 0$ to time $t = 2.0\,\text{s}$.

(a(ii))[1]

Describe qualitatively the change, if any, in the frequency of the sound heard by the child that comes from the car horn, from time $t = 4.0\,\text{s}$ to time $t = 6.0\,\text{s}$.

(b)[2]

Determine the frequency, to three significant figures, of the sound heard by the child that comes from the car horn at time $t = 3.0\,\text{s}$.

(c)[2]

Determine the time needed for the sound emitted at time $t = 4.0\,\text{s}$ to reach the child.

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