A stone is thrown vertically up from the tower of height 25m with a speed of 20 m/s What time does it take to reach the ground ? `(g=10 m//s^(2))`

Explanation

Sign convention must be used to solve this problem. It is shown in figure. We consider the upward direction as positive and downward direction as negative with respect to a point of reference. In the above example the point of projection is considered as the point of reference.
Then, u=20 m/s
`a=g=-10 m//s^(2)`
`s=h=-25 m`
From equation of motion `s=ut + 1//2 at^(2)`
`-25 =20 t-1//2 x 10 x t^(2)`
`-25=20t -5 t^(2)`
`-5=4t -t^(2)`
`rArr t^(2)-4t-5=0`
Solving this equation,
We get, (t-5) (t+1)=0
t=5 or -1
t=5s