Question

An object of mass $10kg$ is moving with an initial velocity of $2m{s}^{-1}$. To make it come to rest in one second, what should be the unbalanced force?

Answer 1

Initial velocity $u=2m{s}^{-1}$
velocity at rest $v=0m{s}^{-1}$
time interval $t=1$ second
mass $m=10kg$
change in velocity in one second or acceleration

$a=\frac{\text{Final velocity - Initial velocity}}{t}$

$a=\frac{0-2}{1}$

$\Rightarrow a=2m{s}^{-1}$

Apply newton's second law
$F=ma$
$F=10\times (-2)$
$20kgm{s}^{-2}$ or $-20$ newton or $-20N$. The negative sign tells us that the force exerted is opposite to the direction of motion.