Question

A child is moving on a skateboard at the speed of $2\frac{m}{s}$. After a force acts on him, his speed becomes $3\frac{m}{s}$. Find the work done by the external force on the child, given the weight of the child, is 20 kg.

• A. 1 J
• B. 10 J
• C. 20 J
• D. 320 J

Answer 1

Answer: (B)

50 J

Let initial velocity of the child be $v1$ and final velocity be $v2$
$v_1=2m/s$
$v_2=3m/s$
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.
Work done by the external force on the child $=\frac{m{v}_2^2}{2}–\frac{m{v}_1^2}{2}$

$=\frac{20\times (9–4)}{2}$

$=10\times 5=50J$