Question

When a source of frequency f₀ moves away a stationary observer with a certain velocity an apparent frequency f ' is observed. When it moves with the same velocity towards the observer, the observed frequency is 1.2 f '. If the velocity of sound is v, then the actual frequency f₀ is –

• A. $\frac{12}{11}f'$
• B. $\frac{11}{12}f'$
• C. $\frac{7}{6}f'$
• D. $\frac{6}{7}f'$

Answer 1

Answer: (A)

$\frac{12}{11}f'$

Answer 2

f ¢ = f₀ $\left( \frac { v } { v + u } \right)$ and 1.2 f ¢ = f₀$\left( \frac { v } { v - u } \right)$

\ u = $\frac { \mathrm { v } } { 11 }$

\ f₀ = f ¢ $\left( 1 + \frac { \mathrm { u } } { \mathrm { v } } \right)$ = $\frac { 12 } { 11 } \mathrm { f } ^ { \prime }$