A ball thrown vertically upwards with a speed of 19.6 ms−1 from the top of a tower returns to the earth in 6s. Find the height of the tower.
(Take g=9.8 m/s2)
A
27.8m
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B
58.8m
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C
70m
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D
92m
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Solution
The correct option is B58.8m
Sign convention: Taking upward positive.
Given that,
Initial velocity of ball, u=19.6 ms−1
Acceleration due to gravity, g=−9.8ms−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=58.8m
Hence, option (b) is correct.