A ball is projected upwards from the top of a tower with a velocity `50ms^-1` making an angle `30^@` with the horizontal. The height of tower is 70m. After how many seconds from the instant of throwing, will the ball reach the ground. `(g=10 ms^-2)`

Options

A. 2s
B. 5s
C. 7s
D. 9s

Answer

C

Explanation

`s_(y) = u_(y)t + (1)/(2) a_(y)t^(2)` or `- 70 = 25t - 5t^(2)`
`rArr 5t^(2) - 25t - 70 = 0`
`rArr t^(2) - 5t - 14 = 0`
`rArr t^(2) - 5t - 14 = 0`
`rArr t^(2) - 7t +2t - 14 = 0`
`rArr r(t-7) +2 (t-7) = 0`
`rArr t = 7, t =- 2`
`:. t = 7s`