Question 162: Waves

Question

The driver of a bus approaching a big wall notices that the frequency of his bus's horn changes
from 420 Hz 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 ms–1.

Accepted Answer

Frequency received by wall,
$${f_w} = \left( {{{330} \over {330 - v}}} ight){f_0}$$
Frequency after reflection, $$f' = \left( {{{330 + v} \over {330}}} ight){f_w}$$
$$ = \left( {{{330 + v} \over {330}}} ight) 	imes \left( {{{330} \over {330 - v}}} ight){f_0}$$
$$ \Rightarrow $$ $$490 = \left( {{{330 + v} \over {330 - v}}} ight)420$$
$$ 	herefore $$ v = 25.2 m/s
= 91 km/h

Answer

91 kmh–1

Answer

81 kmh–1

Answer

61 kmh–1

Answer

71 kmh–1

Waves

The driver of a bus approaching a big wall notices that the frequency of his bus's horn changes from 420 Hz 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 ms–1.

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