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

`v_(y) = - 50 sin 30^(@) =-25 m//s`
`h = ut + 1/2 "gt"^(2) rArr 70 =-25 t + 1/2 xx 10 xx t^(2)`
`rArr 5t^(2) - 25t - 70 =0`
`rArr t^(2) - 5t - 14 = 0`
`t = 7,-2`
`therefore t = 7`sec.