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Question

A construction worker’s helmet slips and falls down when he is 78.4 m above the ground. He hears the sound of the helmet hitting the ground 4.23 seconds after it has slipped. Find the speed of sound in air. ( Acceleration due to gravity 9.8m/s2 )



A

512 m/s

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B

215 m/s

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C

341 m/s

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D

651 m/s

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Solution

The correct option is B

341 m/s


Let t be the time taken by the helmet to reach the ground.
Here, u = 0, h = 78.4 m and g = 9.8 m/s2.
h=ut+12gt278.4=12×(9.8)×t2
t2=2×78.49.8t=4 sec
 The time taken by the helmet to drop to the ground = 4 s.
The time taken by sound to travel 78.4 m=(4.234) s=0.23 s
The speed of sound in air=78.4 m0.23 s=340.87 m/s341 m/s


Physics

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