Question

A ball is thrown vertically upwards with a velocity of $20m{s}^{-1}$ from the top of a multi storey building.The height of the point where the ball is thrown 25 m from the ground.How long will it be before the ball hits the ground ? Take $g=10m{s}^{-2}$.

• A. t=5s
• B. t=10s
• C. t=15s
• D. t=20s

Answer 1

The correct option is A t=5s

Time to reach maximum height can be obtained from $v=u+at$
$0=20+(-10)t$
$t=2s$
$s=ut+0.5a{t}^2=20\left(2\right)+0.5(-10)(2{)}^2=20m$
Thus, total distance for maximum height is 45 m
$s=ut+0.5a{t}^2$
$45=0+0.5\left(10\right)(t^{\prime }{)}^2$
$t^{\prime }=3s$
Total time= 3+2= 5s