Question

A source of sound $A$ emit waves of frequency 1800 Hz is falling towards ground with a terminal speed $v$. The observer $B$ on the ground directly beneath the source receives waves of frequency $2150Hz$. The source $A$ receives waves, reflected from ground, of frequency nearly :
(Speed of sound $=$ 343 m/s)

• A. $2150Hz$
  
• B. $2500Hz$
  
• C. $1800Hz$
  
• D. $2400Hz$
  

Answer 1

The correct option is B $2500Hz$

From Doppler's effect, we know

$f_o=\frac{(v+v_o)}{(v+v_s)}{f}_s$

$\frac{343}{343+v_s}=\frac{2150}{1800}as{v}_o=0\left(i\right)$

Now reflected wave will become source with velocity vs and frequency

2150Hz and A will become observe. Therefore,

$f_o^{\prime }=\frac{343}{343+v_s}\times 2150\left(ii\right)$

From i and ii

$f_o^{\prime }=2150\times 2150/1800=2568Hz$

Frequency is approx $2500Hz$.