Question

A child is moving on a skateboard at the speed of $2m{s}^{-1}$. After a force acts on him, his speed becomes $3m{s}^{-1}$. Calculate the work done by the force, if the weight of the child is $20kg$.

• A. $20J$
• B. $50J$
• C. $100J$
• D. $320J$

Answer 1

The correct option is B

$50J$

Let initial velocity of the child be $v_1$ and final velocity be $v_2$
$v_1=2m{s}^{-1}$
$v_2=3m{s}^{-1}$
Since the child is in motion, he will have a kinetic energy of $\frac{m{v}^2}{2}$, which will be increased when an external force acts on him.
Work done by the external force on the child $=\frac{m{v}_2^2}{2}–\frac{m{v}_1^2}{2}$

$=\frac{20\times (9–4)}{2}$

$=10\times 5=50J$.