Question Detail
Question
A car with a siren of frequency 280 Hz is moving towards a stationary observer at 5 m/s. Calculate the frequency observed by the man before and after the car passes.
Correct Answer
Option A is the correct answer.
Detailed Explanation
Before car passes (approaching): f' = f v/(v - v_s) = 280 × 340/(340 - 5) = 280 × 340/335 = 284.18 ≈ 285 Hz. After car passes (receding): f' = f v/(v + v_s) = 280 × 340/(340 + 5) = 280 × 340/345 = 275.94 ≈ 276 Hz. The closest is 285, 271 Hz.
Hint
Use the Doppler effect formula for a moving source: f' = f v/(v ± v_s).