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ICSE Class 10 Physics 2013 Paper

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Question : 20 of 70
Marks: +1, -0
A sound made on the surface of a lake takes 3 s3\text{ s} to reach a boatman. How much time will it take to reach a diver inside the water at the same depth?
Velocity of sound in air =330 ms−1=330\,\text{ms}^{-1}.
Velocity of sound in water =1450 ms−1=1450\,\text{ms}^{-1}.
Solution:  
   Depth of lake     =v×t\;\text{ Depth of lake }\;\; = v \times t
  =330×3\; = 330 \times 3
  =990 m\; = 990 \text{ m}
Time taken by sound in water
  =   depth of lake      velocity of sound in water   \; = \frac{\;\text{ depth of lake }\;}{\;\text{ velocity of sound in water }\;}
  =9901450\; = \frac{990}{1450}
  =0.68 sec.\; = 0.68 \text{ sec}.
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