Question

A ball is projected vertically up with a velocity of $20m{s}^{-1}$. Its velocity, when the displacement is $15m$, is......... $m{s}^{-1}.$ (Take $g=10m{s}^{-2}$)

• A. $10$
• B. $15$
• C. $-10$
• D. Both (A) and (C)

Answer 1

Answer: (C)

$-10$

Given:

Initial velocity $u=20m/s$

Diaplacement $s=15m$

Gravitational acceleration $g=10m/s^2$ (Considering downard direction as positive)

Final velocity $v=?$

Using the equation of motion , $v^2=u^2-2gh$
$v^2={20}^2-2\times 10\times 15$

$\Rightarrow v^2=100$

$\Rightarrow v=\pm 10m/s$
Since the upward direction is negative, the required velocity is $v=-10m/s$