Question

(a) How long will a stone take to fall to the ground from the top of a building $80m$ high and (b) what will be the velocity of the stone on reaching the ground? (Take $g=10m{s}^{-2}$)

• A. (a) $14$ $s$, (b) $30$ $m$ $s^{-1}$
• B. (a) $4$ $s$, (b) $30$ $m$ $s^{-1}$
• C. (a) $10$ $s$, (b) $40$ $m$ $s^{-1}$
• D. (a) $4$ $s$, (b) $40$ $m$ $s^{-1}$

Answer 1

Answer: (D)

(a) $4$ $s$, (b) $40$ $m$ $s^{-1}$

a.) $s=ut+0.5a{t}^2$

$80=0+0.5\left(10\right)t^2$
$t=4s$
b.) $v=u+at$

$=10\times 4=40m/s$