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?
- A.0.085 km
- B.0.85 km
- C.0.17 km
- D.1.7 km
Correct answer
Explanation
The correct answer is B, 0.85 km. In 5 seconds the sound travels to the mountain and back, a total of 340 × 5 = 1,700 m, so the mountain is half of that, 850 m or 0.85 km, away.
An echo is the sound heard again after it is reflected from a large surface such as a cliff, a mountain or a tall building. Because the sound makes a round trip, the distance to the reflecting surface is d = (v × t) ÷ 2, where v is the speed of sound and t is the time between producing the sound and hearing the echo. Our ear keeps a sound for about 0.1 second, so in air at about 340 m/s the reflecting surface must be at least about 17 m away for a distinct echo to be heard.
A is wrong because 0.085 km, or 85 m, is ten times too small. C is wrong because 0.17 km comes from multiplying 340 by 0.5 instead of 5 seconds. D is wrong because 1.7 km is the total distance the sound travels there and back, not the distance to the mountain.
Exam tip: for an echo, distance = speed × time ÷ 2; the minimum distance for a clear echo in air is about 17 m.
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