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Q13 (CDS-II/2019) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Waves and acoustics Answer Verified

An echo is heard after 5 seconds of the production of sound which moves with a speed of 340 m/s. What Is the distance of the mountain from the source of sound which produced the echo?

Result
Your answer: —  Â·  Correct: B
Explanation

To calculate the distance of the mountain, we must account for the fact that an echo involves sound traveling to the obstacle and back to the source [1]. The total distance covered by the sound wave is the product of its speed and the time taken [1]. Given the speed of sound is 340 m/s and the time delay is 5 seconds, the total round-trip distance is 340 m/s × 5 s = 1700 meters. Since this distance represents the path to the mountain and back, the actual distance to the mountain is half of the total distance: 1700 m / 2 = 850 meters. Converting this value into kilometers (850 m / 1000), we get 0.85 km. This principle of using time delay to find distance is a fundamental application of wave reflection.

Sources

  1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 8: Measurement of Time and Motion > 8.3 Speed > p. 113
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