A car starts from rest and accelerates at 5m s ^ - 2 , att = 4s , a...

velocity of car at $t = 4sec$ is

$v = u + at$

$v = 0 + 5 ( 4 )$

$= 20m{s}^{- 1}$

At $t = 6sec$

acceleration is due to gravity $∴ a = g = 10m{s}^{- 2}$

${v}_{x} = 20m{s}^{- 1}$ (due to car)

${v}_{y} = u + at$

$= 0 + g ( 2 )$ (downward)

$= 20m{s}^{- 1}$ (downward)

$v = \sqrt{{v}_{x}^{2} + {v}_{y}^{2}}$

$= \sqrt{{20}^{2} + {20}^{2}}$

$= 20\sqrt{2}m{s}^{- 1}$