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NDA 2017 GAT · General Knowledge · Sound and Waves · medium

An echo is heard 0.5 s after a sound is produced. If the speed of sound in air is 340 m/s, the distance of the reflecting surface is

Single correct option

An echo is heard 0.5 s after a sound is produced. If the speed of sound in air is 340 m/s, the distance of the reflecting surface is
  1. A. 170 m
  2. B. 340 m
  3. C. 42.5 m
  4. D. 85 m
Show answer and solution

Answer: (D) 85 m

The sound travels to the obstacle and back, so the total path is speed times time = 340 x 0.5 = 170 m. The reflector is at half of this distance, 85 m. Giving 170 m forgets that the path is a round trip. For a clear echo the minimum distance is about 17 m for a 0.1 s delay.
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Original question written in the NDA 2017 GAT pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake