An echo returned in 2s. What is the distance of the reflecting surface from the source, given that the speed of the sound is 342 ms-1.

  1. 342 m
  2. 648 m
  3. 171 m
  4. 85.5 m

Answer (Detailed Solution Below)

Option 1 : 342 m
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The correct answer is 342 m.

CONCEPT:

  • Echo: If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo.
    • Echoes are heard due to the phenomenon of Reflection of sound waves.
    • To hear the echo clearly, the reflecting object must be more than 17.2 m from the sound source for the echo to be heard by a person standing at the source.

CALCULATION:

Speed of sound = 342 m/s.

The time is taken for hearing an echo = 2s.​

  • The speed of sound is

\(⇒ Speed (v)= \frac{distance (d)}{time (t)}\)

Distance travelled = 2d = v × t

⇒ 2 × d = 342 × 2

⇒ d = (342 × 2)/2

⇒ d = 342 m
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