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MHT CET Medical202623 April 2026Morning ShiftPhysicsWaves and SoundActual

A train moves towards a stationary observer with speed 20 m/s. The train sounds a whistle and its frequency heard by the observer is f₁ . When the train moves away from the platform at a speed of 20 m/s, the frequency heard is f₂ . If the speed of sound is 340 m/s, then f₂ f₁ is

Options

  1. A7 8
  2. B8 7
  3. C9 8
  4. D8 9

Correct answer

D. 8 9

Step-by-step solution

Using Doppler's effect, the apparent frequency when the source moves towards a stationary observer is given by: f₁ = f₀ ( v v - v_s ) Substituting v = 340 m/s and v_s = 20 m/s: f₁ = f₀ ( 340 340 - 20 ) = f₀ ( 340 320 ) When the source moves away from the stationary observer, the apparent frequency is given by: f₂ = f₀ ( v v + v_s ) Substituting the values: f₂ = f₀ ( 340 340 + 20 ) = f₀ ( 340 360 ) Taking the ratio f₂ f₁ : f₂ f₁ = f₀ ( 340 360 ) f₀ ( 340 320 ) = 320 360 = 8 9 Answer: 8 9

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