Question

A sounding body of negligible dimension emitting a frequency of 150 Hz is dropped from a height. During its fall under gravity it passes near a balloon moving up with a constant velocity of 2 m/s one second after it stared to fall. The difference in the frequency observed by the man in balloon just before and just after crossing the body will be :
(Given that - velocity of sound $=300m/s;g=10m/s^2$)

• A. 12
• B. 6
• C. 8
• D. 4

Answer 1

Answer: (A)

12

after $1sec,$ falling point sound source $($with freq $150Hz)$ will attain velocity of $V_s=10m{s}^{-1}$, and observer sitting on the balloon is coming upward with velocity $V_o=2m{s}^{-1}$, say frequencies observed

by observer before and after crossing are $n_1$ and $n_2$

respectively, then

$n_1=\frac{300+2}{300-10}150=156.20Hz$
$n_2=\frac{300-2}{300+10}150=144.19Hz$

$n_1-n_2=12Hz\left(approx\right)$.

option "A" is correct.