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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 m
Here 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

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