A train is moving on a straight track with speed 20 m s - 1 . It is blowing its whistle at the frequency of 1000 Hz . The percentage change in the frequency heard by a person standing near the track as the train passes him is (speed of sound = 320 m s - 1 ) close to:

Step-by-step solution

Before train passes f ′ = v v - v s × f 0 = 320 320 - 20 × f 0 = 320 300 × f 0 = 16 f 0 15 After train passes f ′ ′ = v v + v s × f 0 f ′ ′ = 320 320 + 20 f 0 = 320 340 f 0 = 16 17 f 0 Change in frequency = 16 15 - 16 17 f 0 ∴ Percentage change in frequency = 16 15 - 16 17 f 0 f 0 × 100   ≈ 12 % nearly