Question

A ball is thrown vertically upwards which attains a maximum height of $20m$. Calculate the velocity with which the ball is thrown upwards? (Take $g=9.8m{s}^{-2}$)

• A. $29.79m{s}^{-1}$
• B. $17.79m{s}^{-1}$
• C. $19.79m{s}^{-1}$
• D. $15.79m{s}^{-1}$

Answer 1

The correct option is C $19.79m{s}^{-1}$

Let initial velocity of ball be $u$
Distance travelled, $s=20m$
Final velocity, $v=0m{s}^{-1}$
Acceleration due to gravity, $g=9.8m{s}^{-2}$
Acceleration of the ball, $a=-9.8m{s}^{-2}$
($\because$ motion is in upwards direction)
Using third equation of motion,
$v^2$ = $u^2$ + $2as$
$0=u^2+2\times (-9.8)\times 20$
$u^2=392$
$⟹u=19.79m{s}^{-1}$
Hence, the ball is thrown with velocity $19.79m{s}^{-1}$