Question

# A ball thrown vertically upwards with a speed of 19.6 ms−1 from the top of a tower returns to the earth in 6 s. Find the height of the tower. (Take g=9.8 m/s2)

A
27.8 m
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B
58.8 m
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C
70 m
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D
92 m
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Solution

## The correct option is B 58.8 m Sign convention: Taking upward positive. Given that, Initial velocity of ball, u=19.6 ms−1 Acceleration due to gravity, g =−9.8 ms−2 Time taken by ball to return ground, t=6 s Let the height of the building is h Net displacement, s=−h By using the 2nd equation of motion s =ut+12gt2 Substitute values ⇒−h = 19.6×6+12×(−9.8)×62 ⇒−h= 117.6−176.4 ⇒−h=−58.8 ⇒h = 58.8 m Hence, option (b) is correct.

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