A stone dropped from the roof of a building takes 4 s to reach the ground. The height of the building is.

9.8m

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19.6m

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39.2m

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78.4m

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Solution

The correct option is D 78.4m
The formula for time of flight is $t = \sqrt{\frac{2 h}{g}}$
where h is the height from which ball is dropped.
since, $t = 4 s$
Putting the value in above formula,
$4 = \sqrt{\frac{2 h}{9.8}}$
$h = 7.84 m$

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