Question 385: Waves

Question

A car sounding a horn of frequency $$1000 \mathrm{~Hz}$$ passes a stationary observer. The ratio of frequencies of the horn noted by the observer before and after passing the car is $$11: 9$$. If the speed of sound is '$$v$$', the speed of the car is

Accepted Answer

Frequency of a source moving towards a stationary listener is $$\mathrm{n}_{\mathrm{b}}=\left(\frac{\mathrm{v}}{\mathrm{v}-\mathrm{v}_{\mathrm{s}}}ight) \mathrm{n}$$
Frequency of a source moving towards a stationary listener is $$\mathrm{n}_{\mathrm{a}}=\left(\frac{\mathrm{v}}{\mathrm{v}+\mathrm{v}_{\mathrm{s}}}ight) \mathrm{n}$$
Taking the ratio
$$\frac{n_b}{n_2}=\left(\frac{v+v_s}{v-v_s}ight)$$
$$\begin{aligned}
& \frac{11}{9}=\left(\frac{v+v_s}{v-v_s}ight) \
& 11 v+11 v_s=9 v-9 v_s \
& 2 v=20 v_s \
& v_s=\frac{1}{10} v
\end{aligned}$$

Answer

$$\mathrm{V}$$

Answer

$$\frac{\mathrm{v}}{2}$$

Answer

$$\frac{\mathrm{v}}{5}$$

Answer

$$\frac{\mathrm{v}}{10}$$

Waves

A car sounding a horn of frequency $1000 \mathrm{~Hz} passes a stationary observer. The ratio of frequencies of the horn noted by the observer before and after passing the car is 11: 9. If the speed of sound is 'v$', the speed of the car is

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