COMEDK Exam 2014 Solved Paper

Given, velocity of sound, v=330m∕s
Let frequency of horn be f and speed of the car be vc.
The frequency of the hom of the car heard by the policeman before it crosses him is given as
f′=f(

v

v−vc

) ................(i)
and after it crosses him is given as
f′′=f(

v

v+vc

)................(ii)
From Eqs. (i) and (ii), we get

f′′

f′

=

v−vc

v+vc

=

330−vc

330+vc

⇒

f′′

f′

=

330−vc

330+vc

...............(iii)
Since, f′′=f′−15%off′=0.85f′
∴ From Eq. (iii), we get

0.85f′

f′

=

330−vc

330+vc

⇒ 0.85=

330−vc

330+vc

⇒vc=26.7ms−1