A ball is relased from the top of a building `180m` high. It takes time t to reach the ground With what speed should it be projected down so that it reaches the ground time `(51)/(6)`.

Options

A. `50 ms^(-1)`
B. `61 ms^(-1)`
C. `11 ms^(-1)`
D. `2 ms^(-1)`

Answer

C

Explanation

`180 = (1)/(2) at^(2)`
`t = sqrt((360)/(g)) = 6`
`t' = 5`
`180 = 5u + (1)/(2) xx 10 xx 5`
`u = 11 m//s`