A ball is thrown vertically upwards with a velocity of `20ms^-1` from the top of a multistorey building. The height of the point from where the ball is thrown is 25.0 m from the ground.
How long will it be before the balls hits the ground?
Take `g=10ms^-2` [Actual value of `g` is `9.8ms^-2]`.

Explanation

Time spent in air (t) is `y_1-y_0=V_0t+1/2 g t^2`
where `y_1`= Total displacement of the body from ground=0
`therefore0=y_0+V_0t+1/2g t^2=25+20t-1/2. 10.t^2`
[`because g=-10m//s^2` while going up]
`therefore0=-5t^2+20t+25(or)t^2-4t-5=0`
i.e., `(t-5)(t+1)=0impliest=5(or)t=-1`
But time is not -ve.
`therefore` Time spent in air t= 5 sec