    Question

# A ball is thrown vertically upwards with a speed of 9.8 m/s from the top of tower. If it returns to ground in 3 seconds, find the height of tower. (Take g=9.8 m/s2)

A
58.6 m
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B
14.7 m
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C
18.9 m
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D
24.3 m
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Solution

## The correct option is B 14.7 mHere we take the refrence level at the top of the tower. Figure given below shows, the motion scenario of ball. Initial speed of ball: u=+9.8 m/s Displacement of ball when it reaches ground: S=ut+12at2 ...(i) Putting S=−h, a=−9.8 m/s2 in Equation (i). We are taking the reference +ve axis as per shown in figure. Hence, a=−g=−9.8 m/s2 ⇒−h=(9.8)×t+12(−9.8)t2 ...(ii) Putting t=3 s, we get: −h=(9.8)×3+12(−9.8)32 ∴ h=14.7 m downward, as measured from top Hence option B is the correct answer  Suggest Corrections  0      Similar questions  Related Videos   Motion Under Gravity
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