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Question

# A stone is dropped into a 200 m deep well, if the well is dry, after how much time the sound of stone hitting the bottom will be heared. (speed of sound in air =340 m/s and g=10 m/s2)

A
6.91 s
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B
8.52 s
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C
11.43 s
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D
2.51 s
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Solution

## The correct option is A 6.91 s Depth of the well, h=200 m Speed of sound, v=340 m/s Let time taken by stone to reach the bottom of the well be t1, so, we have s=ut1+12at21 ⇒ 200=0+12×10×t21[∵ s=h,u=0 and a=g] ⇒ t1=2√10=6.32 s Now, time taken by sound to reach the mouth of the well t2=hv t2=200340=1017 s=0.59 s Time after which sound is heard is t=t1+t2=6.91 s

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