A ball 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//sec`
B. `25 m//sec`
C. `30m//sec`
D. `35 m//sec`

Answer

C

Explanation

A ball is ………..
Let initial velocity of the ball `= u`
`:. 25 = ut - (1)/(2)"gt"^(2)= ut-5t^(2)`
`implies 5t^(2)-ut+25=0`
If `t_(1)` and `t_(2)` are the two solutions,

Then, `t_(2)-t_(1) = sqrt((t_(2)+t_(1))^(2)-4t_(1)t_(2))`
`implies 4^(2)=(u^(2))/(25)-4xx(25)/(5) implies 16^(2) = (u^(2))/(25) - 20`
`implies u^(2)=25xx36 implies u = 30 m//s`.