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  • The frequency of a car horn encountered a change from 400 Hz to 500 Hz. When the car approaches a vertical wall. If the speed of sound is 330 m/s. Then the speed of car is ______ km/h.Option: 1 132Option: 2 -Option: 3 -

The frequency of a car horn encountered a change from 400 Hz to 500 Hz. When the car approaches a vertical wall. If the speed of sound is 330 m/s. Then the speed of car is ______ km/h.
 

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Frequency of a car horn cncounter change from 400 Hz to 500 Hz

\begin{aligned} &f_{\text {app }}=500 \mathrm{~Hz}\\ &f=400 H z \\ &f_{\text {app }}=f\left(\frac{V+V_{s}}{V-V_{s}}\right) \\ &\frac{5}{4}=\frac{\left(330+V_{s}\right)}{\left(330-V_{s}\right)} \end{aligned}

\begin{gathered} 5 \times 330-5 V_{s}=4 \times 330+4 V_{s} \\ 330=5 V_{s}+4 V_{s}=9 V_{s} \end{gathered}

V_{s}=\frac{330}{9} \mathrm{m} / \mathrm{s}

\begin{aligned} V_{s}=\frac{330}{9} \frac{\mathrm{m}}{\mathrm{s}}=\frac{330}{9} \times \frac{18}{\mathrm{5}} \\ \left(\because 1 \frac{\mathrm{km}}{\mathrm{h}}=\frac{5}{18} \frac{\mathrm{m}}{\mathrm{s}}\right)\\ =132\left(\frac{\mathrm{km}}{ \mathrm{hr}}\right) . \end{aligned}

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