Question

A person feels 2.5% difference of frequency of a motor-car horn. If the motor-car is moving to the person and the velocity of sound is 320 m/sec, then the velocity of car will be

• A. 8 m/s (approx.)
• B. 800 m/s
• C. 7 m/s
• D. 6 m/s (approx.)

Answer 1

Answer: (A)

8 m/s (approx.)

Answer 2

By Doppler’s formula $n' = \frac{nv}{(v - v_{S})}$

Since, source is moving towards the listener so $n' > n$.

If n = 100 then n' = 102.5

⇒ $102.5 = \frac{100 \times 320}{(320 - v_{S})}$ ⇒ $v_{s} = 8m/\sec$