Answer: Option B
:
B
Let initial velocity of the child be $v_1$ and final velocity be $v_2$
Given, $v_1=2m{s}^{-1}$, $v_2=3m{s}^{-1}$ and massof the child $=20kg$.
Since the child is in motion, he will have a kinetic energy of$\frac{m{v}^2}{2}$, which will be increased after an application of force by an external agent on him.
Now, Applying work energy theorem ,
Work done by the external force on the child $=\frac{m{v}_2^2}{2}–\frac{m{v}_1^2}{2}$
$=\frac{20\times (9–4)}{2}$
$=10\times 5=50J$