A ball is thrown upwards from the top of a tower `40 m` high with a velocity of `10 m//s.` Find the time when it strikes the ground. Take `g=10 m//s^2`.

Explanation

In the problem `u=+10m//s, a=-10 m//s^(2)` and `s=-40 m`
(at the point where ball strikes the ground)
Substituting in `s=ut+(1)/(2)at^(2)`
`-40=10t-5t^(2)-10t-40=0` or `t^(2)-2t-8=0`
Solving this we have t=4s and -2s. Taking the positive value t=4s.