A policeman on duty detects a drop of $10 \%$ in the pitch of the hom of motion of car as it crosses him. If the velocity of sound is $330 \,m / s$. Calculate the speed of the car ........... $m / s$
Medium
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(a)
drop q $10 \% \Rightarrow$ heard frequemy was $90 \%$ of original so if orisinal $=\nu$ then apparent $=0.9 \nu$ usins $\nu^{\prime}=\nu\left(\frac{v}{v+v s}\right) \quad$; vs $=\frac{\text { speed of }}{\text { car. }}$ put $\nu^{\prime}=0.9 v$
we qet $0.9=\frac{v}{v+u s} \Rightarrow 0.9 v+0.9 v s=v$
or $\quad 0.9 v_s=0.1 V \Rightarrow v_s=\frac{V}{9}$
put $v=330 \,m / s \quad U_s=\frac{330}{9}=36.7 \,m / s$
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