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Question

# A ball thrown vertically upwards with speed 'u' from the top of a tower reaches the ground in 9 s. Another ball is thrown vertically downwards from the same position with speed 'u'. take 4 s to reach the ground. Calculate the value of 'u'. (Takeg=10ms−2)

A
15 ms1
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B
20 ms1
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C
25 ms1
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D
45 ms1
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Solution

## The correct option is C 25 ms−1Ball is thrown from the top of tower, ⇒height=−h, For the first case, velocity = u , Time t=9s, Using s=ut+12at2 ⇒−h=u×9−12×10×92⇒−h=9u−405 .....1For the second case, velocity = −u , Time t=4s, Using s=ut+12at2 ⇒−h=−u×4−12×10×42⇒−h=−4u−80 .....2Equating equations 1 and 2, ⇒9u−405=−4u−80 ⇒u=25ms−1

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