Question

A whistle sends out 256 waves in a second. If the whistle approaches the observer with velocity \(\left(\frac{1}{3}\right)^{rd}\) of the velocity of sound in air. The number of waves per second observer will receive is

• A. \(384\)
• B. \(192\)
• C. \(300\)
• D. \(200\)

Hint:

For a moving source and a stationary observer, use f' = f v / (v - vs).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
When a source approaches a stationary observer, the apparent frequency is \(f'=f\,\frac{v}{v-v_s}\).

Step 2: Substitute Values
\(f=256\) Hz and \(v_s=\frac{v}{3}\).

Step 3: Calculation
\(f'=256\times\frac{v}{v-\frac{v}{3}}=256\times\frac{3}{2}=384\) Hz

Step 4: Compare the options
The observer receives \(384\) waves per second, option (A).

Final Answer:
The observer receives 384 waves per second. \[ \boxed{\text{(A) }384} \]