Two cars are moving on two perpendicular roads towards a crossing with uniform speeds of $72\; km/hr$ and $36\; km/hr$. If first car blows horn of frequency $280\; Hz,$ then the frequency of horn heard by the driver of second car when line joining the cars make $45^o$ angle with the roads; will be .... $Hz$
Diffcult
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(b) Here ${v_A} = 72\,km/hr = 20m/\sec $
${v_B} = 36km/hr = 10m/\sec $
$n' = n\left( {\frac{{v + {v_B}\cos {{45}^o}}}{{v - {v_A}\cos {{45}^o}}}} \right)$
$ \Rightarrow $ $n' = 280\,\left( {\frac{{340 + 10/\sqrt 2 }}{{340 - 20/\sqrt 2 }}} \right) = 298\,Hz$
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