Question

A child is moving on a skateboard at the speed of $2m/s$. When a force is applied to him, his speed becomes $3m/s$. Find the work done by the external force on the child, given the weight of the child is 20 kg.

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

Answer 1

The correct option is B

50 J

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