Answer
a)$\vec v =-24.4 m/s$
b)$t =4.53 s$
Work Step by Step
lets take upward direction as positive.
Thus we have
$\vec u = 20m/s \\ \vec s = -10m \\ \vec a =-g =-9.8m/s^2 \\ \vec v =? \\ t =?$
a) We know that
$2\vec a. \vec s = v^2 - u^2$
$2*(-9.8)*(-10) = v^2 - 20^2$
$v^2 =196 +400$
$ v=±\sqrt 596m/s$
since the velocity will be in downward direction. Therefore
$\vec v =-24.4 m/s$
b) We have
$\vec v = \vec u + \vec a t $
$ -24.4 =20 -9.8t$
$ t =\frac{ 20+24.4}{9.8}$s
$t =4.53 s$
