Question

A child is moving on a skateboard at a speed of 2 m/s. After a force acts on him, his speed becomes 3 m/s. What is the work done by the external force, given the mass of the object is 20 kg?

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

Answer 1

The correct option is B

10 J

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 after the application of force by the external agent on him.
According to work energy theoram,
Work done by the external force on the child = Change in kinetic energy of the child $=\frac{m{v}_2^2}{2}–\frac{m{v}_1^2}{2}$

$=\frac{20\times (9–4)}{2}$

$=10\times 5=50J$