A ball is thrown vertically upwards with a velocity of `20 ms^(-1)` from the top of a multistorey building. The height of the point from where the ball is thrown is 25 m from the ground. How long will it be before the ball hits the ground (Take, `g=10 ms^(-2)`) ?

Explanation

Net displacement , s=-25 m
Negative sign is taken because displacement is in the opposite direction of initial velocity
As `s=ut +(1)/(2) g t ^(2)`
` therefore -25 = 20 t +(1)/(2) xx(-10) xx t^(2)`
` or 5t^(2) - 20 t-25 =0 or t^(2) - 4t -5=0`
`or (t+1) (t-5)=0`
As `t ne -1 , So t=5 s.`