Question

A body of mass $5kg$ is thrown vertically upwards from ground with a velocity of $20m{s}^{-1}$. What will be its momentum just before hitting the ground? Assume upward direction as positive.

• A. $100kgm{s}^{-1}$
• B. $0kgm{s}^{-1}$
• C. $-20kgm{s}^{-1}$
• D. $-100kgm{s}^{-1}$

Answer 1

The correct option is D $-100kgm{s}^{-1}$

Given:
Mass of the body, $m=5kg$
Intial velocity, $u=20m{s}^{-1}$.
We know that $v^2=u^2+2as$
Here, displacement of the body, $s=0$, acceleration, $g=-9.8m{s}^{-2}$
$\therefore$ Velocity of the body just before hitting the ground, $v^2={20}^2+2\times (-9.8)\times \left(0\right)$
$\Rightarrow v=\sqrt{{20}^2}$
Since upward direction is taken as positive, $v=-20m{s}^{-1}$
We know, momentum, $p=mv=5\times (-20)=-100kgm{s}^{-1}$.