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162. Problem 20.71 (RHK) A police car sounding its siren is moving at
( |
and
approaching a stationary pedestrian. The police in the car hear the siren at
12.6 kHz but the pedestrian hears the siren at 13.7 kHz. We have to
find the air temperature. (We can assume that the speed of sound increases
linearly with temperature between
and
). 
and
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Solution: Click For PDF Version From the data we will calculate the speed of sound in air and from the speed we will find the temperature of air. Speed of the police car,
Frequency of the police siren,
Frequency of the siren as heard by the stationary pedestrian, Let the speed of sound be
Relation between the frequencies is given by the Doppler shift relation
From this equation, we get
We will next calculate the coefficient of linear variation of speed of sound
with temperature,
and
Let us assume linear variation of
This gives
Speed of sound of
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.
. We will
use the data
will be
at the air temperature T,