A bal is thrown vertically upwards from the ground. It crosses a point at the height of 25 m twice at an interval of 4 secs. The ball was thrown with the velocity of

Options

A. `20m//s`
B. `25m//s`
C. `40m//s`
D. `30m//s`

Answer

D

Explanation

A ball is ………..
`S = 25m`, u = ?
`t_(Q) = t_(1) =4s`
`a = 10m//s^(2)`
`S = ut + (1)/(2)at^(2)`
`25 = ut + (1)/(2) (-10)t^(2)`
`5t^(2) - ut+25=0`
`t_(1) +t_(2) = (u)/(5)`
`t_(1)t_(2) =5`
now `t_(Q) - t_(1) = sqrt((t_(1)+t_(2))^(2) -4t_(1)t_(2))`
`4 = sqrt(((u)/(5))^(2) - 4xx5)`
`u = 30m//s`