Question

A child is moving on a skateboard at the speed of 2 m/s. After a force acts on him, his speed becomes 3 m/s. Find the work done by the external force on the child, given the weight of the child is 20 kg.
Assume that the surface is on which the child is skateboarding is flat.

• 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 $v_1$ and final velocity be $v_2$

$v_1=2$ m/s
$v_2=3$ m/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.
Now, Applying Work energy theorem
Work done by the external force on the child $W=△KE=K{E}_f-K{E}_i=\frac{m{v}_2^2}{2}–\frac{m{v}_1^2}{2}$

$W=\frac{20\times (9–4)}{2}$

$W=10\times 5=50J$.

The work done by the force on the child is positive since it increases its kinetic energy.