First slide

Doppler's effect

Question

The driver of a bus approaching a big wall notices that the frequency of his bus’s horn changes from 420Hz to 490 Hz when he hears it after it gets reflected from the wall. Find the speed of the bus if speed of the sound is $330 m s^{- 1}$

Moderate

A

91 $k m h^{- 1}$
B

61 $k m h^{- 1}$
C

71 $k m h^{- 1}$
D

81 $k m h^{- 1}$

Solution

$\begin{array}{l} f_{a} = f_{0} \{\frac{C + V}{C - V}\} \\ \Rightarrow \frac{490}{420} = \frac{1 + V / C}{1 - V / C} \\ \Rightarrow V = \frac{330}{13} m / s = 91.38 \frac{km}{h} \approx 91 \frac{km}{h} \end{array}$

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